Actually many G men claim there are mobsters on opposite ends of the spectrum so it's not that silly.
Joe Pistone said Sonny Black was genuinley a nice guy and often went out of his way to help others with no expectation of reciprocation. Granted "nice guys" in the mafia are few and far between and there are likley far more psychopaths than "nice guys".

Accardo had men tortured with blow torches so I wouldn't call him nice.

Columbo was a stone cold killer, one of Profaci's favorite hit men.