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?
Lost to a world in which I crave no part,
I sit alone and listen to my heart,
Pleased with my little corner of the earth,
Glad that I came--not sorry to depart Richard Le Gallienne