iron fist velvet glove

"iron fist in a velvet glove," referring to a person who appears gentle but is determined and often inflexible underneath.  

I used to know a man who appeared gentle but had a vile temper and most definitely an iron fist. Of course when he used his velveted iron fist, and then I winced in painful reaction, he blamed me. Like most psychopaths do. and, fyi, psychopaths are common and not often violent, which is a mistaken image the word psychopath often evokes. They are people who use and manipulate others with no regard for the other's suffering under their charming, velvet covered emotional abuse. So 'iron fist in a velvet glove applies well to psychopaths. And sociopaths, who are just about as bad, although a psychopath seems more unconscious of their deviant behavior. It is deviant to misuse people and behave unkindly while occasionally posing as a friend.

I have a knack for sussing out psychopaths and buying their velvet glove act.