Theres a concept - mainly in conservative circles but still widely believed even outside them - that the rules of war are for chumps and breaking them is something like an instant win button. It just means youre "serious" and are "totally committed to winning"
But the big reason that you have these agreed upon rules of war (they pop up a lot throughout history; ours are just unusual in that they allegedly apply to all nations) is that you otherwise get spirals of vengeance that lead to massacres that do nothing to actually bringing a swift resolution to war (because, while war is awful, if you really must have war it is better to be as short and bloodless as possible).
I remember reading that the London Blitz started off as a mistake - German bombers got lost at night and dropped their payload in a residential district rather than an industrial one. Naturally, Churchill responded in kind. But the Germans did not go "ah, this is retaliation for that thing we did by accident" they went "the nefarious British are terrorizing our people, we must show them we will not be intimidated." Which provoked a similar response ad naseuam.
And the end result is a lot of pointless destruction with no strategic gain.