Fair point. But the last one left is unconsious, and it would be rude to murder it when it's insensible.
It's having bad dreams, poor thing.
And you killed them because they were there. Not for any noble reson, not any defendant's excuse, not for anything that can be named. They were there. You decided that they need not be there. You got rid of them.
It was mostly out of anger, because you were forced from the Sunless City by a superior opponent. Who you are so going to kill permanently.