What would the concrete mental illness be?
Quite possibly some sort of personality disorder, e.g. Narcissistic or Antisocial PD. Most ordinary people are unaware of just how devious and manipulative these types can be. It's like two people in one person, a Jekyll and Hyde personality. A public face of kindness and generosity. And ruthless controlling behavior and manipulation in private, often this behavior is compulsive.
That being said I’m dubious that even a diagnose of NPD or similar is a agreed upon defense against criminal charges.