"Mr. Rogers" refers to Fred Rogers, who had a children's show on PBS for many years called Mr. Rogers' Neighborhood. The show was beloved by parents and children, with kids learning valuable lessons about kindness and acceptance from the genial host.
You would have to have a pretty twisted childhood to have beef with Mr. Rogers, but Korn lead singer Jonathan Davis had a combination of adolescent trauma and drug abuse that led him to lash out at him in this song.
In Mr. Rogers' Neighborhood, folks are generous and kind. Davis learned that this is not true in the real world, and blamed Rogers for leaving him ill-prepared to deal with it, especially when he got picked on in school.
Davis says he got "obsessed" with Mr. Rogers, and spent three months writing this song about what he saw as the TV host's betrayal. The thoughts Davis expresses in his lyrics don't come naturally, of course. He was high on amphetamines while he was writing the song, making Mr. Rogers the target of his drug-induced psychosis.