Comment by UltraSane

Yes. If Ukraine executes Russian POWs, or firebombs Moscow, it's still a war crime even if Russia invaded with the goal of genociding them (yes, saying an ethnicity doesn't really exist and they're just confused Russians, kidnapping children to resettle elsewhere, and forcefully assimilating at gunpoint everyone in the occupied territories is genocide).

War crimes are absolute, there's no "if you weren't first, you get one free".