Elon Musk called Bill Gates ‘categorically insane’ and ‘an a*****e to the core’ after he shorted Tesla stock

As Tesla CEO Elon Musk and Microsoft co-founder Bill Gates have both held the title of the richest man on earth, it’s easy to think that the two would have plenty to talk about.

It could be their love of technology and A.I., perhaps philanthropy, or even the pressures which come with leading a global company.

As it turns out, they appear unable to stand each other.

The Musk vs Gates feud has publicly spilled over a number of times, though 67-year-old Gates has tended to be more complimentary about his rival than his younger counterpart has been about him.

However, a new biography about the Starlink founder appears to have shed more light on the pair’s tumultuous relationship, this time centering around an incident in which Gates shorted Musk-owned Tesla stocks.

Walter Isaacson, Musk’s biographer, wrote the pair met in March 2022 at the Tesla Gigafactory in Austin to talk about philanthropy.

It was during this meeting that the contentious subject of Gates betting against Tesla was brought up: the businessman had lost $1.5 billion by taking a bearish stance against the EV manufacturer.

Gates apologized to Musk for the move, but told Isaacson it did nothing to calm The Boring Company founder.

“Once he heard I’d shorted the stock, he was super mean to me,” Gates said to Isaacson. “But he’s super mean to so many people, so you can’t take it too personally.”

Gates may have a point: Musk has openly threatened to turn up at Meta founder Mark Zuckerberg’s house and fight him, and got into an argument with a disabled former Twitter employee on the social media site now known as X.

According to an excerpt of Isaacson’s biography shared with CNBC, Gates’s justification of the bid against Tesla was that he believed the supply of EVs would soon outweigh demand, and so he would make money by making a bet against Musk’s company.

For Musk, who believes Tesla is among the leading forces against climate change, Gates betting against an EV company while being an advocate for the environment is unforgivable.

Isaacson, whose book is due to be published September 12, wrote Musk asked him after the meeting: “How can someone say they are passionate about fighting climate change and then do something that reduced the overall investment in the company doing the most? It’s pure hypocrisy.

“Why make money on the failure of a sustainable energy car company?”

A couple of weeks following the meeting, Gates reportedly texted Musk to follow up on some philanthropy proposals. Musk asked Gates if he still held a shorted position on Tesla, which the latter confirmed he did.

As a result Musk shut the conversation down, saying he couldn’t take Gates’s work on climate change seriously. The he messaged Isaacson: “At this point, I am convinced that he is categorically insane (and an asshole to the core). I did actually want to like him (sigh).”

For Gates’s part, calling Musk “mean” is about as personal as the billionaire has got.

On the other hand, Musk has insulted everything from Gates’s intelligence to his appearance—last year, around the time of this reported controversy, Musk tweeted a photo of Gates comparing his figure to that of a pregnant person.

Neither Musk, approached via the X press office, nor Gates, approached via the Bill and Melinda Gates Foundation, responded to Fortune when contacted for comment.

‘He’s overboard on Mars’

Isaacson’s account of the meeting also highlights a number of other areas where the pair disagree.

While Gates has reportedly donated more than $50 billion of his fortune through the Bill and Melinda Gates foundation, Isaacson writes that Musk had put $5.7 billion into a charitable fund for tax reasons.

Philanthropy is “bulls***” Musk reportedly told Isaacson, saying there’s only 20 cents worth of impact for every dollar donated.

But another major difference, which Gates has highlighted in the past, is Musk’s fixation on moving to another planet—Mars. The SpaceX founder has a long-standing vision of creating a self-sufficient city on the red planet, while Gates is more occupied with fixing the issues on Earth.

“I’m not a Mars person,” Gates reportedly told Isaacson. “He’s overboard on Mars. I let him explain his Mars thinking to me, which is kind of bizarre thinking. It’s this crazy thing where maybe there’s a nuclear war on Earth and so the people on Mars are there and they’ll come back down and, you know, be alive after we all kill each other.”