taking the life of another

with the recent events occurring in Aurora, Colorado and the senseless killing of people you hear about all the time, I started to think about how we look at taking someone's life.  It is a pretty common thing to want justice for the killing of innocent people.  I am sure most of us think the shooter in the Colorado massacre should be put to death.   We all have our own creative methods for carrying that out, no doubt.  I guess as I get older, though, I realize that killing someone else does not bring another person back to life.  People will say that justice is served by killing him anyways.  Every day he lives and breathes and enjoys living is an insult to justice.  ok, I see that, but my loved ones are still gone and there is nothing I can do about it.  I think the only time I could actually consider killing another person would be in self-defense.  So in the case of the gunman in the Aurora theater, I would feel like it was justified to shoot him because you are defending yourself and innocent people in the theater.  Unfortunately, no one was armed, or if they were, they were not able to stop the gunman.
so this brings up an interesting point about who among us is qualified to make a decision to kill or execute another person.  It is a very emotional issue among us.  To kill another person intentionally because they killed another person is kind of a catch-22.  They killed, and they are bad, so we kill, but we are not bad, right? some people would say only God can make that decision, but God is not around in a physical form right now.  He is not in a jury, or in a theater when a gunman goes crazy.  God doesn't take over when someone tries to kill us and we kill in self-defense.  I can hear the voices now saying that "God works through us and we do the right thing" - well ok, maybe.  that is what you believe and I can respect that.  I feel that ultimately we are all from the same source of love and energy and we return there when we die, whether from old age or premature death. 
I honestly don't think that killing anyone just for revenge or justice doesn't really do much of anything except make us feel justified in our anger and fear, but I guess I could probably argue the point either way if I really wanted to.  That is VALIDATION.