Even though we saw Lex be reborned in the finale of the show it wasnt the real Lex Luthor. The real Lex Luthor was the one who lived in Smallville with Clark and the gang for 7 years of the show, the one who was possessed by Zod, the one who killed his father and cloned brother, and the one who tryed to take down clark by taking the fortress down with him. The real Lex Luthor had completely died the moment his truck blew up by Oliver Queen. The Lex Luthor we saw in the finale was still even though completely finished and look like lex and have his memories was still a clone. The clone parts of other Lex's created him and he had no heart. I can give more info if needed but if u need written feedback heres some, "In the 2-hour series finale, it is revealed Lex had used the parts of other clones to create a perfected one; however, the clone lacked a heart. After a Lionel from an alternate reality sacrifices himself to Darkseid, his heart is placed inside the clone allowing him to live. So R.I. H (hell) to the real Lex and hello Lex clone.