This category is for villains who have any type of psychological disorder and can be deemed as mentally unstable or villains suffering from mental disorders such as Posttraumatic stress disorder, Dissociative identity disorder (Dissociative villains), Schizophrenia and psychosis.

In addition, a villain who is perfectly meaningful, coherent, and stable can still be qualified as mentally ill if the evil deeds they perform go beyond the usual psychopathy by being extremely fiendish and malevolent, the reason being that no sane individual would think it acceptable to take villainy to the degree that they do.

These characters are also Tragic, since the trait of being mentally unstable is being unable to tell right from wrong, examples of which include some Yanderes, the Diamond Pig, Bae Su-mi and Anniyan. Nevertheless, some can also be Pure Evil, such as Amon Goeth, Patrick Bateman, Carnage, Edwin Epps, Dr. Robotnik, Frank Booth and the Joker.

Notes

• To qualify as Mentally Ill, a character must have horrible behavior, beliefs, and actions which do not stem from their cultural beliefs unless it severely hinders the character's activities.
• Not all villains are mentally unstable when they first appear. While some are, others who were sane to begin with suffer experiences that causes their sanity to snap, like Andrew Detmer and Lucy. The primary causes for this are prolonged abuse and humiliation. A severe-enough degree of fear can do it too, but less commonly.
• Mentally ill villains can only be Pure Evil if they make it clear that they have an understanding of what is right and wrong and meet all the criteria, even inspite of their limited views on reality.
• Non-humanoid entities and demons almost never count (e.g. Valak), regardless of how evil they can be. To them, such acts are normal or a simple spiritual/cultural function.