He neutralized him with the Krytonium gas doe. Without it, Wonder Woman wouldn't have held him much longer and thus, superman wouldn't have killed him.
If they had replaced Doomsday with Metallo (maybe the guy in the wheelchair being used to make Metallo), delayed Superman's death & not craped out with Lex Luthor (I'm sorry--Alexander) I'd give it an A+
I like it a lot but I'd have to give it a C+. Maybe B.