That's the pacifist answer but no that's not how war crimes work: The rules of war aren't about avoiding bloodshed, they're about avoiding pointless bloodshed, pointless from the point of winning an armed conflict, that is. If you can shorten a conflict and spare millions of lives by killing a couple thousands of civilians, well, a couple thousand is less than millions. War is erm dispassionate like that, a hard-nosed calculus.

Hence why you also get rules like the ban on hollow-point bullets: They're more likely to kill than to disable. Killing combatants, however, is less effective at binding up enemy resources and thus not a sound military strategy, using them means that you care more about killing people than winning the engagement. If, OTOH, the enemy started killing all their wounded soldiers instead of expending medical resources that reasoning would cease to apply and you'd be justified using hollow points. (Which are btw in ample use by police forces because they ricochet much less, leading to less injured bystanders, but you generally don't have bystanders on the battlefield. Similarly tear gas is allowed for police use but outlawed for war because it could get confused with a nasty chemical attack very easily, possibly leading to a very nasty escalation when the attacked force responds in kind. Also for the record there's plenty of legitimate uses of white phosphorous, tracer rounds and smoke screens all use it, the banned use is as an incendiary weapon anywhere close to civilians but that's not special to white phosphorous, that's a general thing about incendiary weapons).

Especially when you've cornered that population in an open air prison before bombing them.

There are better weapons though. Also, shooting people who are trying to evacuate through your lines is generally considered bad. Compressing the population into a smaller area that you're using 2,000 pound bombs in is also bad.

Nobody is expecting zero civilian causalities, but this is obviously the most inept army or a professional army conducting a genocide.