Duke University psychiatry professor Allen Frances claimed President Trump may be responsible for millions of deaths and said he's been more destructive than infamous dictators Adolf Hilter, Mao Zedong, and Joseph Stalin -- a series of claims that sparked criticism over CNN's lack of pushback.

Frances appeared on CNN's "Reliable Sources" Sunday, and almost immediately contradicted himself by comparing Trump to mass murderers, right before he discouraged personal attacks.

"Trump is as destructive a person in this century, as Hitler, Stalin, and Mao were in the last century," he said. "He may be responsible for many more million deaths than they were. He needs to be contained, but he needs to be contained by attacking his policies, not his person."

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Frances said calling Trump crazy would be an insult to the mentally ill, and claimed American voters are insane for wanting to elect him in the first place.

"I’ve known thousands of patients, almost all of them have been well-behaved, well-mannered good people. Trump is none of these," he said earlier in the interview.

"Lumping the mentally ill with Trump is a terrible insult to the mentally ill and they have enough problems," Frances continued. "Calling Trump crazy hides the fact that we’re crazy, for having elected him and even crazier for allowing his crazy policies to persist."

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CNN host Brian Stelter faced backlash on Twitter for not stepping in sooner to challenge Frances' initial comments, and later acknowledged he should have done more.

"I agree that I should have interrupted after that line. I wish I had heard him say it, but I was distracted by tech difficulties (that's why the show open didn't look the way it normally does, I had two computers at the table, etc). Not hearing the comment is my fault," Stelter tweeted.