War makes murderers out of otherwise decent people. All wars, and all decent people.

Benjamin Ferencz (b. 1920) American lawyer, international legal scholar, activist

“What the last Nuremberg prosecutor alive wants the world to know,” interview with Leslie Stahl, 60 Minutes (7 May 2017)
    (Source)

Ferencz served as chief prosecutor of twenty Einsatzgruppen officers during the Nuremberg war crimes trials. Longer excerpt:

STAHL: Did you meet a lot of people who perpetrated war crimes who would otherwise in your opinion have been just a normal, upstanding citizen?
FERENCZ: Of course, is my answer. These men would never have been murderers had it not been for the war. These were people who could quote Goethe, who loved Wagner, who were polite --
STAHL: What turns a man into a savage beast like that?
FERENCZ: He's not a savage. He's an intelligent, patriotic human being.
STAHL: He's a savage when he does the murder though.
FERENCZ: No. He's a patriotic human being acting in the interest of his country, in his mind.
STAHL: You don't think they turn into savages even for the act?
FERENCZ: Do you think the man who dropped the nuclear bomb on Hiroshima was a savage? Now I will tell you something very profound, which I have learned after many years. War makes murderers out of otherwise decent people. All wars, and all decent people.