Olaf modifies himself to be smarter to make Elsa happy. Then he decides to make the whole world better, and Elsa tries to kill him because she thinks he’s some sort of evil manipulator now. He’s set up as a metaphor for robots that can make themselves smarter, which rationalists think will destroy the planet.

I later found out this is the same author as a famous Superman fanfic where Lex Luthor kills Superman because he thinks Superman is bad somehow. I never read it myself, but it’s popular with rationalists.