Horror gets normalized during every war. Do you think WW2 was all happy times and flower circles for 6 years? People got used to losing their family, friends, and neighbours. People got used to thousands dying per day.
Rules in war, lol. Rules don't exist if they can't be enforced and there's nobody that can (and actually wants to) enforce them, even if the ICC was set up to do that. It's toothless, just like the UN. The big players will never respect anything, and even all the little counties combined can't match any of the big counties enough to do anything about it.