Lex Luthor - Moments of Cruelty

Like Magneto, Lex Luthor acts primarily out of passion. When his plans are foiled, Luthor acts radically and cruelly; in Superman: Doomsday he admits "I can cure any case of muscular dystrophy on the planet with one inoculation...take this down to the boys in the lab, I need them to make it into a life time treatment." 

In Action Comics #900 Lex Luthor has the power to bring happiness to every earth citizen but, selfishly, he chooses forfeit that opportunity because he cannot defeat Superman. 

In Superman Annual #5 Lex Luthor murders Sasha Green, a martial arts instructor, because she embarrasses him in front of Supergirl and Lois Lane.