1.) Super-heroes are largely defined by their villains. Especially their arch-enemies. Which is why Lex as mis-guided good guy does not work.
2.) "He was a good man, once" actually means something if he is truly evil now.
3.) It's more tragic for Superman. Whenever he confronts his arch-enemy, he remembers the good person who was his best friend. That works better if Lex is basically evil incarnate, as well as someone who can possibly intimidate satan.
4.) A story where Lex does a good deed is more compelling if he is the heart of darkness.