Split Personality Takeover

After months or years of suffering a Split Personality, Enemy Within, or Super-Powered Evil Side, the character is cured! ... by having the weaker, "sane" personality lose all control over the body to their inner evil. Like the slow erosion of a mountain, Jekyll has succumbed to Hyde's implacable id-like force, leaving nothing but the shell of who they were.

This isn't just an identity crisis or something that can be wished away with good intentions, at least not easily. This is the permanent and literal death of a person— or at least a personality.

This is especially common if one personality is more dominant than the other, or the "good" personality is subject to Mind Rape or a Heroic BSOD. Occasionally, a truly strong willed "good" personality can do the takeover— although it's likelier that they'll choose to "merge" or reintegrate with their evil side into a "whole" personality because Good Is Impotent.

Examples

The Suffering: Ties that Bind has the bad ending where Blackmore, the other personality, wiping out Torque completely (pictured above).

A variation occurs in Elfen Lied as the good personality takes over the evil (if tragic and even pitiful) one.

Happens to The Count in Gankutsuou, who can almost claim without lying that That Man Is Dead.

Marik and Yami Marik from Yu-Gi-Oh!. The split personality was born when Marik's father carved his back. The pain was so severe that Marik created a second masochistic personality to cope with it. Yami Marik took control to kill his father a year later and intended to do away with the rest of the family, but he retreated back into Marik's subconscious due to his Morality Chain adoptive brother. When Marik's brother is possibly fatally harmed, Yami Marik takes control and nearly succeeds in killing Marik for ownership of his body.

X-Men: Evolution, episode Sins of the Son. Charles Xavier discovers he has a son in Scotland, David Haller, and goes to meet him, only for two other boys to enter the fray as well. These three boys (David, Ian, and the evil Lucas) are all parts of the same being. Xavier tries to help expunge Lucas, but ends up locking away David, forever losing his son as Lucas goes off, never to be seen again (as the series was cancelled before this story could continue).

In The Amazing World of Gumball episode "The Name", when Gumball's discovery of his first name being Zach results in "Zach" becoming his own person, the latter tries to alter the former's memories to erase him from existence.

Pretty Sammy quotes this trope to Misao which it encourages her to help her in getting rid of Ramia's evil influence over her.

• Even if you stay as Pixy Misa forever, I'll still really love you, Misao!

Psycho has it in the end where it's revealed that Norman Bates' "Mother" personality has taken over, and that Norman is talking in his mother's voice and his thoughts are her voice.

This happened to Dr. Jekyll at the end of his strange case. Luckily for Victorian England, Jekyll saw this coming, locked himself up before changing completely, and ultimately committing suicide.