What would you do if you knew God was dead and the Devil was real?

It wouldn't affect me at all, honestly. I'd probably continue to live exactly the same way. As far as I can tell, there isn't any difference between God and the Devil in the first place, except they fought and one of them won while the other didn't. They both treat humans like shit. So there would be no difference at all.

Going to have to learn more about who these figures are. If god is dead, then he had an end. If he had an end then he cannot be timeless. If he's not timeless he had to have a beginning. If he had a beginning, something must have existed before him. If something must have existed before him, then most of theology is thrown out the window and the very basis if religious understanding becomes broken at a fundamental level.