Question doesn't make any sense to me since killing sometimes involves justice like in the area of war or for righteous cause against a terrorist or maniac. Also, your form of justice might not be the same as everyone else's justice. The very concept of justice is vague since it involves everyones own interpretations of correctness.
My take on this isn't too complicated. If you take a life purposefully then it's only fitting that your life should end as well. I see no other way around it, and I don't like the idea of a murderer getting a chance to keep on living.
When it comes to rapists and pedophiles, I don't feel like they should die because they didn't necessarily take any lives, but they do deserve the life of prison for a very long time.
Revenge is a tricky topic, but it is understandable to a degree. You better bet your ass that if someone took the life of someone that I care about very much that I would want them dead in a heartbeat. I never understood that process of "If I kill this person for revenge then that makes me as bad as them" mentality. That bastard should have never took a life away from me, so why can't I be the one to end his life? Is it selfish?
It really depends on the specific crime involved but most of the time I only go for killing if the person has killed a person or an abundant amount of people.
