The idea that since someone deserves punishment anything is justifies, it’s barbaric.
I think that one significant change happened in recent history which had a big impact. That is the atomic bomb. It is the first time that humans were awe struck at their own power of destruction. Up until then, they viewed the world as big enough to handle killing your enemies and gaining their territories. But now we suddenly felt responsible for human preservation.
The Romans would have the same reaction had they been facing the potential extinction of the human race.
Another thing is that only a society that feels safe can afford to entertain ideas of civil treatment of POWs. Even then, at the hint of a threat the whike facade is up in smoke.