Like MsAnne, I'm not entirely sure about self defense. I am simply not a violent person by nature, and I can only remember getting truly angry in cases where a seemingly innocent third party had been wronged. Still, I can conjecture that if my life were genuinely threatened, I would kill.
I do, however think that there are some crimes that, if witnessed, would drive me to kill. If I were to witness a rape, especially of someone who I especially feel inclined to defend, I'm almost certain that, without much contemplation at all, I would kill the offender. Similarly, there are some forms of cruelty that would almost certainly drive me kill if I were witness to them. A third, and in my opinion much more ambiguous, situation is basically the gross disregard of human dignity. In the massacre of wounded knee, for instance, it was reported that some of of the murderers disrespected dead women and children in ways that would most certainly get me in some trouble if I were to describe them here. Even though no person was actually in physical pain due to these abominable acts, I don't know what I would do if I were there, especially considering that the defilers were said to gloat over their tragic crimes.
This really is a sad question to answer.
-jon