Near the beginning of “Where’s My Roy Cohn?” the new documentary about the lawyer and power broker who mentored Donald Trump, an interviewee says, “Roy Cohn’s contempt for people, his contempt for the law, was so evident on his face that if you were in his presence, you knew you were in the presence of evil.” He wasn’t being hyperbolic.

The film, which opens in New York and Los Angeles on Friday, will likely be of wide interest because of how Cohn helps explain Trump. In the attorney’s life, you can see the strange ease with which a sybaritic con man fit in with crusading social reactionaries. You see the glee Cohn derived from being an exception to the rules he enforced on weaker people. From him, Trump learned how, when he was in trouble, to change the subject by acting outrageously, to never apologize and always stay on the offense. When the Justice Department claimed that apartment buildings owned by the Trump family were discriminating against black renters, it was Cohn’s idea to countersue the Justice Department for $100 million.

In the 1950s, as chief counsel to Senator Joseph McCarthy, Cohn wasn’t just a key player in the anti-Communist witch hunts of the time. He also persecuted men in the State Department who were suspected of being gay, despite being a closeted gay man himself. Later, he became a consigliere to New York’s mafia families, some of whom also had ties to Trump, even as he ranted about law and order.

The film’s title comes from something Trump said when he was frustrated with then Attorney General Jeff Sessions. Cohn was Trump’s template for what a lawyer is supposed to be. (In Attorney General Bill Barr, he seems to have found someone who satisfies him.) “Roy was somebody that had no boundaries,” a lawyer in his firm says in the film. “And if you were on the right side of him, it was great. And if you were on the wrong side of him, it was terrible.”