Lex Luthor - Superman Vol. 1 #149

In Superman #149 (Vol. 1) Lex Luthor discovers "element z" while serving time in prison. Luthor asks the prison warden for 24 hours in the prison laboratory so that he can try to develop a cure for cancer. Luther succeeds in discovering a cure and becomes a hero to the whole nation- prompting even Superman to support his early release. 

Out of prison, and set up as a legitimate scientist, Luthor begins receiving threats and attacks from other criminals. The criminals want Luthor to stop doing good deeds. Seeing the influx of attacks, Superman builds Luthor a space station where he can conduct his experiments. 

Superman receives a distress call from Luthor's satellite and flies right into a trap. Luthor shoots kryptonite rays at Superman and makes Lois Lane and Jimmy Olsen watch him suffer. Luthor then administers a lethal dose of kryptonite into Superman's blood... killing him.