What I really know about technically "mentally ill" I know from watching Law and Order and staying in a Holiday Inn Express.
However, from a layman's perspective:
* He did some extremely irrational things. There was a huge amount of paranoia. While it is true that he hammered down others in his org, took it over, and profited from it enormously, great success in a madman's endeavor does not equate to sanity. Neither does being a powerful orator.
* He suffered from both a massive ego, the inability to see through his own errors, and debilitating substance abuse.
* His relationship with women was both strained, held in shadowy secrecy, and manifestly perverse.
* His teachings and edicts, based on hate for almost all other peoples outside his own organization, have caused great harm to millions.
* When he died, his own followers largely either failed to honor him, or were kept in the dark as to his funeral and burial.
Whoops - silly me! Was I talking about the "Judge" Rutherford, or Adolph Hitler???