There have been a number of Superman gone bad stories over the years, but Kurt Busiek and Walt Simonson's "The Beast of Krypton" reads as one of the best (in large part because it's by Kurt Busiek and Walt Simonson). Superman #666 (cover dated October, so it's all Halloween all over the place) presents a Kryptonian demon who followed Kal-El's rocket when Kryptonian Hell blew up along with the planet, and he's been more than a little starving waiting for the only compatible soul to get corrupted. Since that's Superman's, it wasn't going to happen without some help. So the demon does get that help from some evil god-things, and plans to corrupt Kal in his dreams (it counts!). Thankfully, the Phantom Stranger hatches a plan to create a soul-shell for Superman that CAN be corrupted, while his real soul remains inside, free from taint. And that soul-shell is a real bastard. After making an inappropriate comment about Zatanna's astral "can", he goes on to kill all his enemies. My favorite death is Lex Luthor's:
Once they're all dead, Superman goes to town on his supporting cast. Again, I've got to have a favorite bit and it's Jimmy's poetic end:
Teach ya to turn that signal watch on just to impress the ladies! After that, it's pushing Abraham Lincoln from his seat to make himself an orbital throne from which to shoot down Earth's missiles, then making Smallville slip into Hell, until he meets his own personal demon, who turns him into a "Beast". In the interest of showing every possible look Superman's ever had (which is our thing here at Reign of the Supermen Central), here it is:
As you can imagine, that's when Superman's real soul punches out of the shell and brokers a deal with the Lords of Hell that leaves the Krypto-demon powerless. Looks like Clark doesn't remember much from his dream afterwards, which seems a mercy given that he killed his pregnant wife and everyone he knew in it. That's some rough REM sleep!