Trump is manipulative, superficially charming, dishonest, capricious; he never, never takes responsibility for mistakes. He uses people, then casts them aside with scarcely a thought.
Of course many people in politics are like this to one degree or another.
Jon Ronson, among others, has suggested that psychopaths often achieve success in business.
Of course I don't know if Trump is truly a psychopath (which is actually a personality disorder, not a mental illness), but I wouldn't be surprised, or even hold that against him.
And, of course, that wouldn't make him immune from senile or pre-senile dementia.