I agree with Slon on this one.

Doing so much killing for so long, and literally looking into the faces of those you kill. Not defending your Homeland/family but being a part of a conquering army.
No chivalry, no honor, no class.
Duty ? I'm sure he would've been shot if he didn't do it, but there is a mental line you need to decide for yourself, as to how aggressively/joyfully you go about it.

If he survived, he probably worked in a slaughterhouse, with a box-cutter.

A Beast he was.

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There's one way to find out if a man is honest-ask him. If he says "Yes", you know he's a crook. Groucho Marx