Here:

His overt acts, craven ambition, delusional beliefs, erratic behaviors, perpetual dishonesty, and mental capacities lie so many norms of deviation apart from all his presidential counterparts that he has to be taken as a unique case.

There are other apt observations by historians and political scientists in the linked piece, but the key fact is that he is literally a person without any conscience, who cares not at all about moral or legal limits.