>>150777870
He was super depressed in Fionna and Cake. Torturing Choose Goose to summon Betty, and felt his entire was pointless. I think he's immortal thanks to Betty's wish. Simon's reasoning to be Ice King was just to be useful, and help Fionna. It took divine intervention by Golb to knock some sense into Simon, and moved on.

At least this time, Fionna and Cake would focus on Fionna and Cake, the first season was primarily about Simon.