Psychopathy is a personality disorder involving specific antisocial behaviors. Psychopaths are manipulative and lack the ability to feel either empathy or remorse. They can cause harm to others and to themselves, and the harm they inflict tends to be premeditated against victims who are taken in by their charm. A character with this disorder will likely appear normal to others, which helps them maintain their secret. While psychopathy exists on a spectrum, these individuals aren't often motivated to change because they don't feel guilty about their behavior. Even when they pursue help, successful treatment for psychopathy has been inconsistent at best, making this a convenient disorder for an antagonist, antihero, or a protagonist navigating a failed character arc.

Dark of Disturbing (primary), Illegal or Immoral Acts

BEING JUDGED, BEING LABELED, BEING UNABLE TO ACHIEVE A DREAM, CHANGE, DISCRIMINATION, ISOLATION, LOSING AUTONOMY, LOSING ONE'S SOCIAL STANDING, NOT BEING IN CONTROL, NOT FITTING IN, REJECTION, TRUSTING OTHERS

Keeping a secret comes at a cost, even when it's a positive one. But in the case of a psychopath, the secret doesn't hold them back because they don't feel guilt or shame. They don't care that they're hurting people. Even the usual drawback of a secret keeping the character from getting help is mitigated in this case because treatment for psychopathy is rarely successful. So while this secret can cause great harm for others, keeping it doesn't limit the character in significant ways.

Maintaining a façade of normalcy with a steady job and a circle of acquaintances
Feigning remorse if they do something they know is considered wrong or bad
Pretending to cry when they hear bad news 
Learning the expected emotional responses to life events
Mimicking the emotions of those around them 
Going through the motions of making amends if they've slipped up