To some extent if McKay had no semi automatic handgun it's likely that at least some of his family would have survived that night. Screams usually attract attention I don't think they even had the chance to scream :( 
Pulling a trigger is easy when you're in despair but it's not so simple to stab, gut & cut up adult family members.  It takes too much time, most don't have the stomach for it, someone always gets away or they have a good chance of over-powering you in greater numbers.
Guns are a number eliminator. Especially a semi automatic. He can rip through everyone in seconds. That "tool" is meant to murder and kill. That's what it does. That's what it was created for, you can use it to defend your family and yourself but it's intended to end the life of whoever or whatever you point it at, I really wish he would have just slept off whatever problem he was facing.  I wish he would have asked for help. I wish he hadn't killed the family he was supposed to protect. :( just sad