We never nuked the Nazis...But even if we did, are you saying that the Nazis being Nazis would've justified vaporizing every civilian man, woman, and child in a city or two?
Whether or not you'd say it was justified is a different beast altogether than having to be the one that made it possible in terms of responsibility.
Tangentially, many firing squads will have only one person have a real bullet(s) while the rest have blanks so that they don't all have to feel responsible for ending a life. Even that is setting justification aside.