I think a good point would be to view it in terms of laymen and monks and nuns
Monks and nuns have made a dedication to the precepts and to nibbana, with a renouncement of wordly life, for them, killing wouldn't be an option
However laymen, while accepting the Buddha, have only taken the basic precepts and are very much a part of wordly life and being a part of worldly life would mean taking worldly action, in extreme situation, such as in the case of the Third Reich
I can think of a good thought experiment which is quite widely used in ethical philosophy - If you had a time machine, would you go back and kill Hitler as a baby, or as a teenager or at any point before he joined the German Workers' Party?
“ Your mind is likewise blocked. But the right road awaits you still. Cast out your doubts, your fears and your desires, let go of grief and of hope as well, for where these rule , then the mind is their subject." Boetius