that's a genuinely good question. check out what Chiapas or Rojava did/do instead.

mostly based on mediation, but their worst punishment is exile, which basically means the society at large does not consent to interact with them. which means they have to find a new community where news of their crimes will follow them. or end up living in solitary confinement in the wilderness, which might be a death sentence.

although in a war, you do not want to arbitrarily return prisoners of war. let alone let them go free. and even though they were fash, killing POW is a war crime.