Is it really a war crime to kill a war criminal?
It's not like he had surrendered. He had won. His goal was killing half the universe. He did that. He even took steps to made sure it couldn't be undone.
So it was less like executing a downed enemy soldier and more like assassinating the commander, after he had won the war. You don't get to unilaterally start a war and then get to decide when it's over.
Well I mean, you can, but if you complain about it afterwards you look like a bitch