Elon Musk's biographer saw him fly into 'demon mode.' It's hard to watch — but it also gets stuff done, he says.

• Walter Isaacson said Elon Musk could be "brutal" and would go into "demon mode" at times.
• The biographer said the criticism was productive but made some people afraid to tell him the truth.

Elon Musk's biographer, Walter Isaacson, said the billionaire could go into "demon mode" at times — a state that Isaacson described as highly productive but "dark" and "with a real lack of empathy."

He said during a Twitter Spaces interview on Thursday that the musical artist Grimes, Musk's former girlfriend, coined the term. She told him that it could be "unpleasant" to be around Musk when he's in demon mode but said it's the mode that "gets shit done."

Grimes, whose given name is Claire Boucher, has two children with Musk. Grimes didn't return a request for comment.

Isaacson plans to release his biography on Musk in September. He has written biographies on several innovators, including Steve Jobs and Leonardo da Vinci. He said many brilliant and successful people, including Musk and Steve Jobs, had a "dark streak."

Isaacson said they were not saddled with as much empathy and, as a result, were more able to focus on accomplishing a larger mission.

In Musk's case, the biographer said the billionaire had a "maniacal sense of urgency" that could frighten some of his workers. He said the CEO's demeanor would change when people didn't match his sense of urgency.

"He'd go dark and I'd know that he was just going to rip that person apart," Isaacson said, adding that it was a common occurrence when the billionaire first took over Twitter and gutted over half of the social-media site's staff.

The biographer, who observed Musk's day-to-day life for about two years, said the moments of rage were "uncomfortable" for him to watch.

"He is just brutal," Isaacson said. "The thing that I noticed is that once he finishes doing it — and it was never physical and it was almost done in a flat monotone — but he would just really attack people and then a few days later, if they absorbed the lesson, he'd forget about it. It would be as if he went from becoming Dr. Jekyll to Mr. Hyde and then didn't even think that much or remember that much of how tough he had been on people."

Musk's criticism seemed effective about 80% of the time and was "problematic" 20% of the time, which he said could even make people "afraid to give him bad news," Isaacson said. He said that at times he'd later find out that the man Musk had chewed out had made a mistake because of personal issues, like losing a child two weeks prior.

Isaacson said Musk told him it's a form of "egotism" to want to be empathetic to a single person at the expense of the larger mission. The biographer said his book aimed to show the complexity of Musk and the balance between his "dark streak" and the incredible work he had done at Tesla and SpaceX.

Isaacson attributed Musk's "dark streak" and his willingness to take risks to his relationship with his father as well as the bullying he endured in his youth.

"Elon Musk also has a desire for drama, both in his personal and in his professional life," Isaacson said. "He is somebody who feels most comfortable when he's ordered up a hurricane or surge."

Musk's mother, Maye Musk, expressed concern that her son was "exceptional" but was in "danger" of becoming like his father, Isaacson said.

In her book, Maye Musk described her relationship with her ex-husband as abusive. Musk has said his father, Errol Musk, was "a terrible human being." Maye Musk didn't return a request for comment from Insider.

Errol Musk told Rolling Stone in 2017 that he had never intentionally hurt anyone, excluding one scenario where he said he shot and killed three people who he said illegally entered his house.

Errol Musk denied contributing to Elon Musk's "dark streak" in an emailed statement to Insider, saying that "these days Elon and I are on fairly good terms."

Isaacson is far from the first to point out that Elon Musk can be prone to bouts of anger.

The Tesla cofounder Martin Eberhard previously told Insider that Musk used to scream at him at press coverage on Tesla. And in a previous biography on Musk, The Wall Street Journal's Tim Higgins said the billionaire had a reputation for exploding at top executives and employees on the assembly line at Tesla and had even rage-firing some.

Musk has previously denied accusations of rage-firing employees, calling them "false" on Twitter and saying he gave employees "clear and frank" feedback.

Musk didn't respond to a request from Insider for comment.