The revelation that a main character has Dissociative Identity Disorder (DID) has become a recognisable literary trope, primarily in horror and/or thrillers. In the majority of cases, a character thought to be benevolent (usually the protagonist) is revealed to have a “split personality” attributed to DID caused by extreme trauma. The alter is usually malevolent, and has in fact been the villain all along; a manifestation not only of trauma, but as the original identity’s dark half. The alter was created to protect the original identity (often in childhood or adolescence), but ends up taking over, committing heinous acts. Occasionally, as in Shutter Island (2010) and The Number 23 (2007), the alter is in fact the “good” identity, created to escape the guilt felt after the original identity committed a crime. The emergence of an alter due to DID (the literary version), therefore, complicates ideas of hero/villain and victim/perpetrator. In many cases, the original personality (whether “good” or “bad”) does not appear in the text until the climax, further questioning who is responsible for their malicious acts, and whether their malevolent desires have been created or manifested from some inner evil. An evil alter may be destructive to those around it, despite being originally created to protect the younger self from harm and pain. This essay discusses horror films that utilise villainous alters caused by DID, particularly the character of Kevin/The Beast/The Horde in Split (2016) and its sequel, Glass (2019). While underlining that the version of DID used in these texts is inaccurate, problematic and often harmful, this paper utilises disability and mental health literary tropes in order to discuss the manifestation of evil as a disease (for example, the disabled villain, the psychotic killer, the mentally ill as animal). This section will also examine the supposed physical differences between the alters, a phenomenon begun by Dr. Jekyll/Mr Hyde and continued through to Kevin/The Beast. Malevolence becomes physicalised, making The Beast, and many versions of Hyde, supernaturally strong as well as aligning them with animals. Furthermore, the essay explores the Hero/Villain/Monster framework in order to further discuss the genre’s DID characters. DID characters in horror are simultaneously heroes (particularly if it is the protagonist that is revealed to have DID), villains and monsters due to their status as an uncanny being. In Split and Glass, the monstrous aspect is further established with the emergence of The Beast, which features monstrous powers as well as harmful intentions. Various characters in Split state that DID makes Kevin and the alters “more than a man”, noting his extraordinary potential—and extraordinary threat. But Kevin/the alters’ condition also prompts questions of innocence and the exploration of trauma, putting them in a unique position to explore ideas of evil and the “dark half.”