@Book To explain my point of view I must first make a premise, I live in a state where the death penalty does not exist. I agree with this choice because I find it hypocritical that a state that condemns murder bathes itself in the same guilt, by killing the criminal. It is easy to say that If someone killed someone close to me I would be satisfied to see him rot in prison but I am sure that if it happened I would not be able to think that the punishment is fair, but I would like him to be killed too, probably though, if this happened, that person would become a victim and the people close to him would seek revenge and so an infinite circle. Because hatred always generates more hatred. Theoretically speaking the best solution is prison. The problem however arises when the perpetrator is neither accused nor incarcerated but is free to live normally, you said that then a person would not need justification to kill him, but I do not think that is the case, because the moment you kill him you automatically become guilty. Murder, although in some cases it makes us doubt it, is never the right choice.