We don’t allow ourselves to think too deeply of the most heinous crimes, to look too hard at the ghoulish motives of a Jeffrey Dahmer or even an Adolf Hitler.

“Evil,” we say, as if that covers it, as if that’s enough when all that label amounts to is a decision to not consider pathology, the “how a person got to be that way.”

“Evil,” as Dr. Dorothy Lewis once said in shutting down talk show host Bill O’Reilly, “is NOT a scientific concept.”

She should know. She’s made the study of murderers, mass murderers, from Ted Bundy on down the infamy scale, her psychiatric specialty. And what’s she’s discovered, a link between abuse, brain damage or “abnormalities,” could be changing out understanding of the “monsters” among us.

“Crazy, Not Insane” is the distinction at the heart of the prolific documentary filmmaker Alex Gibney’s latest. It’s his third deep dive doc of 2020. And even though it’s narrower in focus than “Totally Under Control,” his film about the screwups in America’s response to COVID 19, or “Agents of Chaos,” about Trump/Russia collusion and Russian interference in the 2016 presidential election, compared with those works, Gibney’s made a less satisfying film on a more intellectually challenging proposition.

“Murderers are made,” Dr. Lewis maintains, “not born.”

Her decades of work covered sweeping changes in human understanding of the brain and in psychotherapy’s responses to that. Brain scans were brought into the courtroom, by her, to explain the damage this future serial killer suffered when an abusive parent ran over his head, or that abused murderer’s disconnect from reality, thanks to a brain cyst.

“The law has a lot to learn from psychiatry,” Lewis declares. “Instead, psychiatry accepts the law’s definitions” of things like “insanity” and “competence,” as in “competent to stand trial.”

In case after case, many of them in death-penalty-friendly states like Texas and Florida, convicted murderers that in earlier eras we’d have referred to as “barking mad” have been injected or rushed to the chair. Lewis consulted on many of them, most often in service of the defense, and discovered shattered minds and “multiples,” criminals whose multi-personality “dissociative disorder” was so severe that any idea that they knew right from wrong, much less what being executed meant, is laughable.

The killer who saved his dessert from his last meal because he was sure the fellow being executed was another person altogether, and that he’d be around to come back and finish, others whose disconnect from reality was just as vast, keep her awake at nights, Lewis says.

It’s not that she wants them returned to society. “Throw away the key” works its way into several diagnoses. Lewis has struggled to strike a balance between “what we discovered” about a criminal’s pathology “and what the law was willing to accept.”

Much of “Crazy, Not Insane” is Lewis reading from her longhand notes for an upcoming book, or Laura Dern reading her words from previously published work.

We see videotaped prison interview sessions. And Gibney uses animation to flesh out her encounters with “twenty-two serial killers” and “a lot of plain old murderers.”

We hear about the politicization of the death penalty, see her ambushed on the witness stand, unprepared by the tack a defense attorney has taken, hounded for her equivocating way of avoiding absolutes and simple “yes or no” answers.

And a frequent counter to Lewis, Dr. Park Dietz, calls some of her assumptions about the convicted “a hoax,” even as he himself sticks his foot in it as a professional prosecution “insanity defense” debunker.

Maybe he’s jealous because Martin Scorsese and Robert DeNiro didn’t come to him (they went to Lewis) in order for the actor to get a feel for the tics, expressions and interior life of a psychopath when prepping for “Cape Fear.”

Lewis describes her research as “like being a detective,” although the questioning of the many killers she’s interacted with (Arthur Shawcross, Joseph Paul Franklin) is rarely as convincing as the simple brain scans showing marked abnormalities.

That’s an issue with the film, too. Lewis, working with neurologist Jonathan Pincus, wrote “Guilty by Reason of Insanity,” making the case that politically ambitious prosecutors and the baying mobs at executions are in a rush to kill those we should be studying, if not sympathizing with. That’s a hard sell, as indeed is “Crazy, Not Insane.”

But the director of “Taxi to the Dark Side” has once again taken on a complex evil being done in our name, a subject no one really wants to think about, and forced us to consider the many ramifications of making a flippant and terminal judgment on something that demands attention and understanding, in light of what we now know.

MPA Rating: unrated, violent subject matter, profanity

Cast: Dr. Dorothy Lewis, Dr. Catherine Yeager, (attorney) Richard Burr, Dr. Park Dietz and the voice of Laura Dern

Credits: Directed and scripted by Alex Gibney. An HBO Max release.

Running time: 1:57