What sort of mental illness exists in serial killers?
A mix of organic disease with iatrogenic interventions can contribute to mental health issues and thus criminal behaviour: one may take a surgical intervention to ease TLE (temporal lobe epilepsy) as an example. In extremely rare cases this can lead to Kluver-Bucy Syndrome. A syndrome with varying symptoms including sexual dysfunctions. For an example of the complications in law and convictions see:
Devinsky, O and Sacks, O 'Klüver–Bucy syndrome, hypersexuality, and the law'
OCD, Narcissistic Personality Disorder, substance abuse and schizophrenia can exist or co-exist in serial killers.
Any mental illness can exist in serial killers.
Schizophrenia, psychopathic, antisocial, split personality disorder, and narcissistic disorders can be present in serial killers.