I have a friend who is a Dr Who fan. He told me there's an episode where the Dr (Tom Baker) is given the chance to kill an infant Hitler. The Dr flatly refuses as it's still murder. I agree with that. However, the moment Adolf is about to give orders to end lives then he should be taken out.
I think under this condition i would be willing to kill the moment they decide to commit to their plan... as they're on their way to the Manchester Arena with a bag of explosives.