Talk-radio host Hugh Hewitt, who is supporting Donald Trump in this year’s election, was subject to the cruel streak months ago while moderating a GOP debate. “A year ago you told me on my radio show—the audio and transcript are out there on YouTube—that you would release your tax returns,” Hewitt said to Trump. “First of all, very few people listen to your radio show,” Trump replied. “That’s the good news. Let me just tell you—which happens to be true. Check out the ratings.” Trump took pleasure in puffing himself up by being gratuitously cruel.

What quality within him causes him to lash out this way? It was on display again when Trump attacked the wife of his primary opponent Ted Cruz. Here is the tweet he sent:

This is vile behavior.

What kind of person attacks a rival by mocking the appearance of his wife? For the whole of his presidential campaign, Trump has gleefully launched gutter attacks like this. And while a cruel streak directed solely at rivals would hardly be excusable, Trump doesn’t even have that excuse. After Chris Christie endorsed him, Trump attended a fundraiser with the New Jersey governor, and said this to the crowd: “I’m not eating Oreos anymore, you know that—but neither is Chris. You’re not eating Oreos anymore. No more Oreos. For either of us, Chris. Don’t feel bad.”

That’s who Trump is: If he’s in front of a crowd with an ally who has a weight problem, he’ll find an excuse to bring it up, to humiliate the ally, for no apparent reason.

And this penchant for purposeless cruelty goes beyond the political realm. "Heidi Klum. Sadly, she's no longer a 10," Trump said once for no apparent reason, baffling the model. “I've known Donald for many, many years. Personally I don't know why he did it," Klum said. "I don't know what I have to do with a presidential campaign." Imagine knowing someone for years, then having them attack your appearance for no reason on national TV. You’d think they were a sociopath.

The people closest to Trump have painful experience with this same quality. In September 1990, Marie Brenner wrote at length in Vanity Fair about how the billionaire humiliated Ivana Trump.

Conservative writer Mona Charen reflected on the same era in National Review:

I first became aware of Donald Trump when he chose to make cheating on his first wife front-page news. Donald and Ivana Trump broke up over the course of months. Not that divorce is shocking, mind you. Among the glitterati marriage seems more unusual. Nor is infidelity exactly novel. But it requires a particular breed of lowlife to advertise the sexual superiority of one’s mistress over the mother of one’s children. That was Trump’s style. He leaked stories to the New York tabloids about Ivana’s breast implants—they didn’t feel right. Marla Maples, by contrast, suited him better. She, proving her suitability for the man she was eager to steal from his family, told the papers that her encounters with the mogul were “the best sex I’ve ever had.” It wasn’t just Donald Trump’s betrayal that caught my eye, nor just the tawdriness—it was the cruelty.

What kind of person treats the mother of his children that way? Is there anyone to whom he wouldn’t be cruel? In fact, none of the examples offered thus far captures the depths of Trump’s cruelty. Understanding that requires hearing the story of the late Freddy Trump, the candidate’s older brother, who died an alcoholic in 1983. After college, Freddy had tried to join the family business, but his heart wasn’t in it. He became an airplane pilot, showing talent in the profession. When his heavy drinking posed a safety risk, however, he quit, and wound up living in an apartment owned by his father and working on one of his maintenance crews, even as his kid brother Donald began to make a name for himself.