What would you do in this situation? [3.5]

[kamikasei]

ATM we're still in a war so that might not be the wisest decision.

Wars are a great time to depose people so long as you think you can do it without disrupting the chain of command too much. Fake an assassination and become the heroes who stepped in to reluctantly take on the burden of rule to stave off defeat. Then you have the backing of the army too. You can even actually hand over authority after the victory to a civilian government if you feel like it.

We never actually worked for him before, we only met him few times. We worked for a bishop he killed.

You lived under his rule. You accepted his protection in war. His laws protected you from crime. Had he done nothing until now to make you realize he was an unpleasant fellow?

War crimes are against the law.

Whose law? His? A treaty that binds him? If there is a law he's broken, consider prosecuting him on that basis. If the law is that of his own nation, does the rest of the government have ways to censure their ruler? My impression from a quick look at the FR wiki is that they don't...

Could a geas/quest work on him? So he'd come to surrender himself to us after the war?

So you're okay with letting him continue to commit atrocities, so long as you get to punish him afterwards? The victims of the war crimes between now and the war's end don't factor in to your plans? Or do you expect for some reason that all his atrocities are committed now and he's got it all out of his system?