I feel like you’re approaching it from a very narrow position.

“War crimes” will invariably happen as long as there’s any significant force on the side of the insurgents and revolutions turns into a civil war. Civil war isn’t fought by well fed knights in shining armor, it’s an uprising by the long oppressed against their oppressor, and a lot of people would use that to settle their scores in what seems as justified, but still unlawful way. It’ll also be highly constrained by material conditions, meaning that food and goods are likely to be in short supply, giving rise to maradeurship or excessive expropriation for army needs — from both sides. I would expect, like in Russian civil war, there to be groups with different morale levels, from staunch communists to forcibly conscripted pro-monarchy civilians, maybe even squads with ex-cons. They would treat local population differently, including potentially causing whole settlements to see the socialists as thugs (just like the other side propaganda told them!) even if later a court martial sentences them for such behavior. And there are other conditions of war - “cruel weapons” may be the only thing you have to fight because your guns are out of ammo and the supply lines are currently cut. Food supply might be so short one has to choose between everybody starving or killing captives to last longer until the help comes, and then the commander might pretend it was done for the sake of justice. Additionally, cultural conditions of your country may be different from modern liberal beliefs. E.g. public beatings may be a totally accepted regular punishment in your country or some of its regions, and it’ll be very same people fighting for socialism - not the humanists heavily versed in ML theory, but those who grew thinking public beatings are the good thing. And the battlefield is the worst place to debate ethics and philosophy.

Or, maybe not. Then your socialist leaders might be forced to push back on excessive terror by their own forces as that may cause both moral decay and public outrage. They might not be wholly successful with that either, even if they try in their earnest. Terror is, ultimately, just a tool but it’s not that powerful in a protracted war where you need to keep your troops together, and it’s likely to backfire eventually, esp. if you’re close to being crushed. Communists shouldn’t justify their actions by “the state does that” but they might be both coming from the cultural conditions where something is not a war crime but a sad daily occurrence. Some insurgent leaders, however, might justify their actions as such and not be stopped by their comrades.