“He went to work for the I.R.S. because he thought that’s where the money was, but it turned out the money wasn’t there,” the younger Mr. Paddock said. “He went to the aerospace industry but the money wasn’t there either. He went to real estate and that’s where the money was.”

Stephen Paddock began buying and refurbishing properties in economically depressed areas around Los Angeles, teaching himself how to put in plumbing and install air-conditioning. By the late 1980s, “we had cash flow,” said Eric Paddock, who added that he had given his life savings to his older brother to invest and eventually became a partner in his company, because “that’s the kind of guy he was. I knew he would succeed.”

“He helped make my mother and I affluent enough to be retired in comfort,” he said.

With success came a rigidity and uncompromising attitude, along with two failed marriages, both short and childless. Stephen Paddock started gambling. Some who met him described him as arrogant, with a strong sense of superiority. People in his life bent to his will, even his mother and brother. He went out of his way for no one.

“He acted like everybody worked for him and that he was above others,” said John Weinreich, 48, a former executive casino host at the Atlantis Casino Resort Spa in Reno, where he saw Mr. Paddock frequently from 2012 to 2014. When Mr. Paddock wanted food while he was gambling, he wanted it immediately and would order with more than one server if the meal did not arrive quickly enough.

Mr. Weinreich said he would get irritated and “uppity about it.”

Mr. Paddock was uncompromising but he was also smart.

“I would liken him to a chess player: very analytical and a numbers guy,” Mr. Weinreich said. “He seemed to be working at a higher level mentally than most people I run into in gambling.”

Mr. Paddock cherished his solitude, his brother said. In 2003, he got his pilot’s license after training in the Los Angeles area, eventually taking the extra step to get an instrument rating so that he could legally fly in cloudy conditions with limited visibility. He bought cookie-cutter houses in Texas and Nevada towns with small airports so that he could park his planes. He was utterly unremarkable.