This makes me think of the show Dexter. It's about a serial killer who's thing is to kill other serial killers. So if you go with the "would you let hitler live" argument, Dexter is doing the right thing, or even has to do it, even though he himself is a serial killer. So then would someone else be "in the right" to kill Dexter, because after all, he does kill a lot of people? It's a pretty complex grey area.
Or the same could be said about Death Note, if you are familiar with that. Is Kira righteous?
It's hard to say for sure what is right. But I kind of lean towards (at least on good days of hope toward humanity) Vash's style of thought, that all life is valuable. Even the "bad people" have good in them, and they are still at the core, people. People can change and be healed of the pain that leads them do bad things. Everyone has flaws and everyone has the potential to overcome their mistakes.
