"Damn you Hyde! Leave me be!"
~ Jekyll
"Can't you see? you are me!"
~ Hyde

Dissociative villains - formerly known as Villains with Dual Personalities - Characters who have been diagnosed with or display the signs of Dissociative Identity Disorder (also known as Multiple Personality Disorder) - these individuals have a fractured psyche that often makes it appear like they have more than one personality, often one or more of these personalities is seen as malicious or dangerous (at least in fiction). Dissociative Identities can occur from different circumstances: It can occur from a mental illness (e.g Norman Bates), a failed experiment (e.g Green Goblin) or from a trauma (e.g Fliqpy).

Do not add beings who are possessed by demonic entities or aliens (for that use Possessed/Brainwashed) - also do not confuse with Alter Egoes (as alter egoes are simply personas made by villains to disguise themselves, perfectly sane individuals may have alter egoes for a variety of reasons such as keeping their identity a secret - much as superheroes traditionally wore disguises : dissociative characters literally see themselves as two (or more) distinct individuals and are not in control of the changes).

This kind of villains generally don't count as Pure Evil given they have no control over their actions due to their illness. However, some can qualify if A) the alter ego of the character meets all criteria and all heinous crimes are traced to said ego or B) their split personality and the original personality are both/all heinous and shows no regerts towards their crimes, even being capable of knowing what their split personality did.