There's this amazing movie from 1996 called Primal Fear starring Edward Norton and Richard Gere. ***SPOILER ALERT***
So basically this kid is found running from an apartment with blood all over him, and in the apartment is the body of a well-respected priest (or maybe a bishop or something idk). Throughout the movie you find out that the kid has split-personality disorder, and you are led to believe that his split personality is who killed the guy. Come to find out, the priest was forcing him into $exual activities with his girlfriend. So the kid brutally murdered him and then pretended it was because of his other personality.
TL;DR: A kid pretends to have split-personality disorder so he can kill someone.