user13266: How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?
This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting". If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.

user471: 
OP: How can I explain to a non follower of the Dhamma that killing living beings intentionally has mental and physical
consequences?

Here, I assume that the non-follower of the Dhamma might be persuaded by scientific research output and professional opinions in the field of psychology and medicine.
Robert T. Muller, a professor of psychology, wrote in the article "Death Becomes Us: The Psychological Trauma of Killing" (Feb 21, 2014):

Killing is often misrepresented in film as far easier than it is. In
reality, the “duty” is mentally taxing, leaving most soldiers
physically ill in the moment and often haunted by nightmares for a
lifetime. Being responsible for ending the life of another human is a
significant source of trauma ...
Apart from about two percent of individuals classified as
“psychopaths,” who, because of deeply rooted personality flaws are
unphased by the act of killing, most soldiers are unprepared for the
task of ending the life of another human being. Many veterans report
that ending even one life is enough to haunt them with painful
memories and sometimes flashbacks.
Soldiers who have engaged in close combat are left with a much higher
likelihood of developing post traumatic stress disorder then their
counterparts who did not.

Stella Compton-Dickinson, a professional clinical therapist wrote in the article "The Psychology of Killing" (Aug 1, 2017):

My work with mentally disordered offenders who have killed,
demonstrates how instinctual, mindless impulses result in high-risk
behaviours. These become observable in body language and symbolic
gestures, which can be explained and understood.

Dave Grossman, retired U.S. Army personnel and former West Point Psychology Professor, wrote together with Bruce K. Siddle in the article "Psychological Effects of Combat" (1999):

... To truly understand the nature
of this resistance of killing we must first recognize that most
participants in close combat are literally "frightened out of their
wits." Once the bullets start flying, combatants stop thinking with
the forebrain, which is the part of the brain which makes us human,
and start thinking with the midbrain, or mammalian brain, which is the
primitive part of the brain that is generally indistinguishable from
that of an animal. ....
Among Vietnam veterans in the United States, PTSD has been strongly
linked with greatly increased divorce rates, increased incidence of
alcohol and drug abuse, and increased suicide rates.

​The above articles talk about the psychological effect of humans killing humans. What about the psychological effect of humans killing animals?
Ashitha Nagesh wrote in the article "The harrowing psychological toll of slaughterhouse work" (Dec 31, 2017), quoting those who have worked as butchers:

Former slaughterhouse workers who have since spoken out about their
time in the industry have described feeling this change within them,
and experiencing PTSD symptoms for years afterwards.
One hog sticker, Ed Van Winkle ... describing ... the
dissociation slaughterers had to force upon themselves. ...
‘The worst thing, worse than the physical danger [of on-the-job
accidents] is the emotional toll,’ Winkle said. ‘Pigs down on the kill
floor have come up and nuzzled me like a puppy. Two minutes later I
had to kill them – beat them to death with a pipe. I can’t care.’ ...
Another such man was Virgil Butler, who worked in a poultry plant in
the US .....
‘The sheer amount of killing and blood can really get to you after a
while,’ .... ‘Especially if
you can’t just shut down all emotion and turn into a robot zombie of
death. You feel like part of a big death machine. [You’re] pretty much
treated that way as well. ...
.... ‘Many people who do this commit violent acts,’ he wrote. ‘They commit
crimes. People who already are criminals tend to gravitate towards
this job. You can’t have a strong conscience and kill living creatures
night after night.

So, how does this relate to the Buddhist precept of not taking a life? The Buddha taught in SN 42.3:

When a warrior strives & exerts himself in battle, his mind is already
seized, debased, & misdirected by the thought: 'May these beings be
struck down or slaughtered or annihilated or destroyed. May they not
exist.' If others then strike him down & slay him while he is thus
striving & exerting himself in battle, then with the breakup of the
body, after death, he is reborn in the hell called the realm of those
slain in battle."

Also, from AN 10.92:

"When a person takes life, then with the taking of life as a requisite
condition, he produces fear & animosity in the here & now, produces
fear & animosity in future lives, experiences mental concomitants of
pain & despair; but when he refrains from taking life, he neither
produces fear & animosity in the here & now nor does he produce fear &
animosity in future lives, nor does he experience mental concomitants
of pain & despair: for one who refrains from taking life, that fear &
animosity is thus stilled.

And what's the purpose of virtue (which includes the Five Precepts)?
From AN 10.1:

“Thus, Ānanda, (1)–(2) the purpose and benefit of wholesome virtuous
behavior is non-regret; (3) the purpose and benefit of non-regret is
joy; (4) the purpose and benefit of joy is rapture; (5) the purpose
and benefit of rapture is tranquility; (6) the purpose and benefit of
tranquility is pleasure; (7) the purpose and benefit of pleasure is
concentration; (8) the purpose and benefit of concentration is the
knowledge and vision of things as they really are; (9) the purpose and
benefit of the knowledge and vision of things as they really are is
disenchantment and dispassion; and (10) the purpose and benefit of
disenchantment and dispassion is the knowledge and vision of
liberation. Thus, Ānanda, wholesome virtuous behavior progressively
leads to the foremost.”


user-1: As someone who has practiced Zen for 15 years and both hunts and raises most of his own meat, let me put in my two cents on the matter.  
First of all, that person you were talking to is full of shit.  But that's to be expected.  Few modern people realize the deep bond that begins to form between the hunter and his quarry or a farmer and his flock.  It is an interspecies intimacy that most will never know outside of a religious practice like Buddhism.  By the time you find yourself aiming your arrow 20 yards from a deer, most of the time you know where she eats, where she drinks, where she sleeps, and who she hangs out with.  You probably also know how old she is and how old some of her children are.  She probably knows you too.  The same goes for raising animals.  When I kill chickens at nine weeks, I've raised them from the time they were one day out of the egg.  I've fed them, cared for them, kept their shelter clean.  Some will even demonstrate undeniably unique personalities.  When I kill something, the experience is truly face to face.  I'm not killing an animal, I'm killing a unique individual.  And it hurts a little.  That person you were talking to clearly never had a comparable experience.  My guess is that most of the meat he/she has eaten has come in a little shrink wrapped package.  They never felt the warmth of that animal's blood or felt the softness of its fur.  If they had, they'd know full well why so many hunter-gatherer cultures have thanksgiving practices that they perform for the animals they've harvested.        
So what does this mean in the context of Buddhism?  Buddhism is all about developing intimacy.  When we become truly intimate with the world around us, we don't have to raise animals to develop that kind of connection.  We feel it viscerally because we directly experience our unbreakable connection to the world around us.  We feel the bond that the farmer feels and what the hunter feels without needing to become hunters or farmers ourselves.  This isn't mystical nonsense.  The more we practice, the more these feelings arise...ultimately becoming just as obvious to us someone hitting us over the head with a sledgehammer.  Ultimately, when we kill, it is 100% clear that we are killing ourselves.  No one who feels that way can go through that action without experiencing mental or even physical duress.                                     

user8157: 
How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?

Kamma is intention. The intention dictates the mental and physical consequences. 

This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting". 

Correct. They hunted for food; to feed their children; following the cycle of life; how nature created them. 

If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.

The above appears to be your personal point of view rather than a Buddhist point of view. If the above idea was Buddhist, there would be specific quotes from scriptures to support this idea; where the Buddha instructed people to not kill for food. Obviously, in the Buddha's time, people would have asked him the same questions about how to gather food. Obviously, the later monks decided such questions did not warrant inclusion in the scriptures. We know the Buddha never taught vegetarianism because he allowed monks to eat meat from almsfood. The Buddha appeared concerned about killing with wanton violence (rather than about how people feed themselves). 

13266: <p>How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?</p>

<p>This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting". If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.</p>
 471: <blockquote>
<p><strong>OP:</strong> How can I explain to a <strong>non follower of the Dhamma</strong> that killing living beings intentionally has mental and physical
consequences?</p>
</blockquote>
<p>Here, I assume that the non-follower of the Dhamma might be persuaded by scientific research output and professional opinions in the field of psychology and medicine.</p>
<p>Robert T. Muller, a professor of psychology, wrote in the article &quot;<a href="https://www.psychologytoday.com/us/blog/talking-about-trauma/201402/death-becomes-us-the-psychological-trauma-killing" rel="nofollow noreferrer">Death Becomes Us: The Psychological Trauma of Killing</a>&quot; (Feb 21, 2014):</p>
<blockquote>
<p>Killing is often misrepresented in film as far easier than it is. In
reality, the “duty” is mentally taxing, leaving most soldiers
physically ill in the moment and often haunted by nightmares for a
lifetime. <strong>Being responsible for ending the life of another human is a
significant source of trauma</strong> ...</p>
<p>Apart from about two percent of individuals classified as
“psychopaths,” who, because of deeply rooted personality flaws are
unphased by the act of killing, most soldiers are unprepared for the
task of ending the life of another human being. Many veterans report
that <strong>ending even one life is enough to haunt them with painful
memories and sometimes flashbacks.</strong></p>
<p>Soldiers who have engaged in close combat are left with a much higher
likelihood of developing post traumatic stress disorder then their
counterparts who did not.</p>
</blockquote>
<p>Stella Compton-Dickinson, a professional clinical therapist wrote in the article &quot;<a href="https://www.psychreg.org/psychology-of-killing/" rel="nofollow noreferrer">The Psychology of Killing</a>&quot; (Aug 1, 2017):</p>
<blockquote>
<p>My work with mentally disordered offenders who have killed,
demonstrates how instinctual, mindless impulses result in high-risk
behaviours. These become observable in body language and symbolic
gestures, which can be explained and understood.</p>
</blockquote>
<p>Dave Grossman, retired U.S. Army personnel and former West Point Psychology Professor, wrote together with Bruce K. Siddle in the article &quot;<a href="https://www.killology.com/psychological-effects-of-combat" rel="nofollow noreferrer">Psychological Effects of Combat</a>&quot; (1999):</p>
<blockquote>
<p>... To truly understand the nature
of this resistance of killing we must first recognize that most
participants in close combat are literally &quot;frightened out of their
wits.&quot; Once the bullets start flying, <strong>combatants stop thinking with
the forebrain, which is the part of the brain which makes us human,
and start thinking with the midbrain, or mammalian brain, which is the
primitive part of the brain that is generally indistinguishable from
that of an animal.</strong> ....</p>
<p>Among Vietnam veterans in the United States, PTSD has been strongly
linked with <strong>greatly increased divorce rates, increased incidence of
alcohol and drug abuse, and increased suicide rates</strong>.</p>
</blockquote>
<p>​The above articles talk about the psychological effect of humans killing humans. What about the psychological effect of humans killing animals?</p>
<p>Ashitha Nagesh wrote in the article &quot;<a href="https://metro.co.uk/2017/12/31/how-killing-animals-everyday-leaves-slaughterhouse-workers-traumatised-7175087/" rel="nofollow noreferrer">The harrowing psychological toll of slaughterhouse work</a>&quot; (Dec 31, 2017), quoting those who have worked as butchers:</p>
<blockquote>
<p>Former slaughterhouse workers who have since spoken out about their
time in the industry have described feeling this change within them,
and experiencing PTSD symptoms for years afterwards.</p>
<p>One hog sticker, Ed Van Winkle ... describing ... the
dissociation slaughterers had to force upon themselves. ...</p>
<p>‘The worst thing, worse than the physical danger [of on-the-job
accidents] is the emotional toll,’ Winkle said. ‘Pigs down on the kill
floor have come up and nuzzled me like a puppy. Two minutes later I
had to kill them – beat them to death with a pipe. I can’t care.’ ...</p>
<p>Another such man was Virgil Butler, who worked in a poultry plant in
the US .....</p>
<p>‘The sheer amount of killing and blood can really get to you after a
while,’ .... ‘Especially if
you can’t just shut down all emotion and turn into a robot zombie of
death. You feel like part of a big death machine. [You’re] pretty much
treated that way as well. ...</p>
<p>.... ‘Many people who do this commit violent acts,’ he wrote. ‘They commit
crimes. People who already are criminals tend to gravitate towards
this job. You can’t have a strong conscience and kill living creatures
night after night.</p>
</blockquote>
<p>So, how does this relate to the Buddhist precept of not taking a life? The Buddha taught in <a href="https://www.accesstoinsight.org/tipitaka/sn/sn42/sn42.003.than.html" rel="nofollow noreferrer">SN 42.3</a>:</p>
<blockquote>
<p>When a warrior strives &amp; exerts himself in battle, his mind is already
seized, debased, &amp; misdirected by the thought: 'May these beings be
struck down or slaughtered or annihilated or destroyed. May they not
exist.' If others then strike him down &amp; slay him while he is thus
striving &amp; exerting himself in battle, then with the breakup of the
body, after death, he is reborn in the hell called the realm of those
slain in battle.&quot;</p>
</blockquote>
<p>Also, from <a href="https://www.accesstoinsight.org/tipitaka/an/an10/an10.092.than.html" rel="nofollow noreferrer">AN 10.92</a>:</p>
<blockquote>
<p>&quot;When a person takes life, then with the taking of life as a requisite
condition, he produces fear &amp; animosity in the here &amp; now, produces
fear &amp; animosity in future lives, experiences mental concomitants of
pain &amp; despair; but when he refrains from taking life, he neither
produces fear &amp; animosity in the here &amp; now nor does he produce fear &amp;
animosity in future lives, nor does he experience mental concomitants
of pain &amp; despair: for one who refrains from taking life, that fear &amp;
animosity is thus stilled.</p>
</blockquote>
<p>And what's the purpose of virtue (which includes the Five Precepts)?</p>
<p>From <a href="https://suttacentral.net/an10.1/en/bodhi" rel="nofollow noreferrer">AN 10.1</a>:</p>
<blockquote>
<p>“Thus, Ānanda, (1)–(2) the purpose and benefit of wholesome virtuous
behavior is non-regret; (3) the purpose and benefit of non-regret is
joy; (4) the purpose and benefit of joy is rapture; (5) the purpose
and benefit of rapture is tranquility; (6) the purpose and benefit of
tranquility is pleasure; (7) the purpose and benefit of pleasure is
concentration; (8) the purpose and benefit of concentration is the
knowledge and vision of things as they really are; (9) the purpose and
benefit of the knowledge and vision of things as they really are is
disenchantment and dispassion; and (10) the purpose and benefit of
disenchantment and dispassion is the knowledge and vision of
liberation. Thus, Ānanda, wholesome virtuous behavior progressively
leads to the foremost.”</p>
</blockquote>
 -1: <p>As someone who has practiced Zen for 15 years and both hunts and raises most of his own meat, let me put in my two cents on the matter.  </p>

<p>First of all, that person you were talking to is full of shit.  But that's to be expected.  Few modern people realize the deep bond that begins to form between the hunter and his quarry or a farmer and his flock.  It is an interspecies intimacy that most will never know outside of a religious practice like Buddhism.  By the time you find yourself aiming your arrow 20 yards from a deer, most of the time you know where she eats, where she drinks, where she sleeps, and who she hangs out with.  You probably also know how old she is and how old some of her children are.  She probably knows you too.  The same goes for raising animals.  When I kill chickens at nine weeks, I've raised them from the time they were one day out of the egg.  I've fed them, cared for them, kept their shelter clean.  Some will even demonstrate undeniably unique personalities.  When I kill something, the experience is truly face to face.  I'm not killing an animal, I'm killing a unique individual.  And it hurts a little.  That person you were talking to clearly never had a comparable experience.  My guess is that most of the meat he/she has eaten has come in a little shrink wrapped package.  They never felt the warmth of that animal's blood or felt the softness of its fur.  If they had, they'd know full well why so many hunter-gatherer cultures have thanksgiving practices that they perform for the animals they've harvested.        </p>

<p>So what does this mean in the context of Buddhism?  Buddhism is all about developing intimacy.  When we become truly intimate with the world around us, we don't have to raise animals to develop that kind of connection.  We feel it viscerally because we directly experience our unbreakable connection to the world around us.  We feel the bond that the farmer feels and what the hunter feels without needing to become hunters or farmers ourselves.  This isn't mystical nonsense.  The more we practice, the more these feelings arise...ultimately becoming just as obvious to us someone hitting us over the head with a sledgehammer.  Ultimately, when we kill, it is 100% clear that we are killing ourselves.  No one who feels that way can go through that action without experiencing mental or even physical duress.                                     </p>
 8157: <blockquote>
  <p><strong>How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?</strong></p>
</blockquote>

<p>Kamma is intention. The intention dictates the mental and physical consequences. </p>

<blockquote>
  <p><strong>This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting".</strong> </p>
</blockquote>

<p>Correct. They hunted for food; to feed their children; following the cycle of life; how nature created them. </p>

<blockquote>
  <p><strong>If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.</strong></p>
</blockquote>

<p>The above appears to be your personal point of view rather than a Buddhist point of view. If the above idea was Buddhist, there would be specific quotes from scriptures to support this idea; where the Buddha instructed people to not kill for food. Obviously, in the Buddha's time, people would have asked him the same questions about how to gather food. Obviously, the later monks decided such questions did not warrant inclusion in the scriptures. We know the Buddha never taught vegetarianism because he allowed monks to eat meat from almsfood. The Buddha appeared concerned about killing with wanton violence (rather than about how people feed themselves). </p>
