the most moral thing to do would be to kill him right when you discover he is the demon lord, every day he isnt dead is a night he spends massacring innocent civilians
Viewing post in Is it possible to go corrupt and kill Richard?
worth it to save the masses of whatever the place is called again
and I could see a way to get away with it, ask him to walk home with you, pretending your going to submit to his demands, and then when no one is around kill him and make it seems like a mugging gone wrong, call the cops yourself from a hiding place and you should be okay