As the American financial system collapsed in the fall of 2008, Stephen K. Bannon began to fantasize about destroying something else: the elite economic and political establishment that he believed had created the crisis.

Mr. Bannon, who was named Donald J. Trump’s campaign chief on Wednesday, was at the time a highly improbable revolutionary, a wealthy former Goldman Sachs banker and a budding filmmaker. But his blue-collar Southern roots tugged at him: panicked by the swooning market, his father, a telephone company lineman with no college degree, had sold much of the stock in his retirement account.

“Steve felt it was outrageous,” said Scot Vorse, his former business partner and a longtime friend.

It was the start of a remarkable reinvention that turned a polished corporate dealmaker who once devised $10 billion mergers on Wall Street into a purveyor of scorched-earth right-wing media who dwells in the darker corners of American politics.

The website he runs, Breitbart News, recently accused President Obama of “importing more hating Muslims”; compared Planned Parenthood’s work to the Holocaust; called Bill Kristol, the conservative commentator, a “renegade Jew”; and advised female victims of online harassment to “just log off” and stop “screwing up the internet for men,” illustrating that point with a picture of a crying child.