user4928: I once read a commentary by a Rinpoche talking about how the karma of killing a single fly grew at an exponential rate. 
Since negative karma grows so quickly, what actions, and what amount of those actions, makes rebirth in a hell realm inevitable? 

user43: Five offences in Buddhism which are said to irrevocably propel the offender into hell are:

patricide,
matricide,
killing an Arhat,
injuring a Buddha,
creating schism in the Sangha

Smaller offenses like common murder, adultery etc. can still be "repaired" even if with great difficulties.

4928: <p>I once read a commentary by a Rinpoche talking about how the karma of killing a single fly grew at an exponential rate. </p>

<p>Since negative karma grows so quickly, what actions, and what amount of those actions, makes rebirth in a hell realm inevitable? </p>
 43: <p>Five offences in Buddhism which are said to irrevocably propel the offender into hell are:</p>

<ol>
<li>patricide,</li>
<li>matricide,</li>
<li>killing an Arhat,</li>
<li>injuring a Buddha,</li>
<li>creating schism in the Sangha</li>
</ol>

<p>Smaller offenses like common murder, adultery etc. can still be "repaired" even if with great difficulties.</p>
