This category is for any villain who has any type of psychological disorder and can be deemed as mentally unstable, or any villain suffering from mental disorders such as 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 are 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 Bae Su-mi, most Yanderes, and Anniyan. Nevertheless, some can also be Pure Evil such as the Joker, Isaac Ray Peram Westcott, Yuuki Terumi, Immortan Joe, Hoyt Volker, The Governor, Kefka Palazzo, Henry Evans, Esther Coleman, and Zant.

Note: 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. Also, if a villain is an Egomaniac or a Spoiled Brat than not being the best at something, having their opinions/desires challenged, or not getting their own way can cause a mental snap. Also, while psychopaths/sociopaths does qualify as a mental illness, not all villains who have mental illnesses are psychopaths/sociopaths, so if you have a villain that is either explicitly stated to be that or otherwise has been clearly depicted as being that, use the category Psychopath instead.