Split Personality Takeover/Playing With

Basic Trope: A character succumbs to his/her dark side.
 * Straight: Jason becomes a monster named Johnny for the rest of his life.
 * Exaggerated: After having enough of being tormented by Johnny, Jason has decided to drink a potion with an ingredient that leads him to become Johnny for the rest of his life.
 * Inverted: Split Personality Merge
 * Johnny the monster becomes Jason, a more friendly character.
 * Justified: Jason suffered a pretty nasty incident, leaving him to believe that "Jason' is dead. Johnny takes over from here.
 * Subverted: It seems like Johnny tries to take over Jason, but the latter is resisting...
 * Double Subverted: ...only to give up at some point.
 * Parodied: Johnny is a Harmless Villain, so Jason lets him take over so he and his friends could have some fun.
 * Deconstructed: Johnny effectively ruins Jason's life, hurting or even killing his close friends(unaware of his other half), getting him in jail, and becoming a poster child for Complete Monster in-universe. Johnny feels like he's done and lets Jason take over from here, forcing him to live with consequences.
 * Reconstructed: Abuse from said consequences makes Jason to get Johnny back in action, the latter agrees, realizing that prison walls are very boring. Johnny escapes and continues his life as a villain.
 * Zig Zagged: "Oh no, Johnny takes me over! Wait, he isn't... he IS! Oh, he isn't..."
 * Averted: Jason keeps his evil side in check, and full takeover is not listed in Johnny's plans.
 * Enforced: "We're out of villains. Wait, we got Jason's internal struggle against Johnny! Let a bad side win it."
 * Lampshaded: "It's the other Jason, the one we don't like!"
 * Invoked: After a takeover, every time Johnny meets any Jason's friend, he brags about it.
 * Defied: Jason acts like he has an evil personality, but actually, HE is a villain in disquise. Suppoused takeover is a part of a plan that benefits him either way. If his friends try to beat Johnny out of him he can pretend it worked and milk the incident for sympathy for a later Evil Plan. If they abandon him, he can guilt them with it, and more importantly, he can stop pretending to be good because Being Good Sucks.
 * Discussed: "I hope my evil alter ego won't take me over and try to kill all of you"
 * Conversed: "Is it really that hard to get rid of him? Call a therapist or something"