It's too easy to hate, and too hard to forgive
ppl who did these atrocities, were mostly heavily mentally influenced by the phenomenon war. War is a mental disease, a shortcoming. When a war begins, suddenly the almost genetical cultural aspects of humans, as feelings towards other ppl are kinda disabled. A human becomes a thing. This happens to two sides and they influence each other to hate the other side, like a flywheel. Hatred of other the other side's committed atrocities, initiating even more gruesome atrocities being committed.
Instead of the self-loving thought "I would never do such a thing, so punish that monster as gruesome as possible", better calm ppl down with reason when you see it happening to begin, or offer a place for refugees. Or try to flee the place asap if you want to stay alive.
No, for real prevention of war (and the atrocities committed in a war) the male part of the lower class, with lower EQ and IQ should be castrated ;-)
oh, and btw, death penalty?
never. (mostly for the reasons mentioned by Karlos)