Lex Luthor - Superman: New Krypton (Story Arc)
In the Superman: New Krypton story arc, Lex Luthor destroys the planet New Krypton using a reactron as a bomb. Destroying an entire planet full of people is a pretty evil deed in itself... but we gain further insight into the mind of Lex Luthor during General Zod's attack on Metropolis in Superman: Last Son.
During the attack, Lex kills a kryptonian solider in combat. His thoughts? "Feels good."
This moment shows Lex Luthor evolving from his role as protector of the human race into something more menacing, violent, and evil. If Luthor was that satisfied after murdering one kryptonian, imagine how he felt after destroying an entire planet of them...