He described wandering through Kenosha with his rifle as protesters set fires and the night grew increasingly tense. He testified that he assisted people with minor medical problems, including a woman, he said, whose ankle he wrapped. He said he also helped put out a fire behind a church.

And he spoke of his first encounters with Mr. Rosenbaum, 36, one of the men he would later kill.

“Mr. Rosenbaum was walking with a steel chain, and he had a blue mask around his face, and he was just mad about something,” Mr. Rittenhouse said, adding that Mr. Rosenbaum screamed that he would kill Mr. Rittenhouse if he found him alone.

Despite the tense situation, Mr. Rittenhouse said he continued to walk through the area asking demonstrators — many of whom had been tear-gassed or suffered scrapes and minor injuries that evening — if they needed medical help.

“I hear somebody scream, ‘Burn in hell,’ and I reply with, ‘Friendly, friendly, friendly,’ to let them know, hey, I’m just here to help,” he said. “I don’t want any problems. I just want to put out the fires if there are any.”

When, close to midnight, Mr. Rittenhouse was chased by Mr. Rosenbaum and later by other people in the crowd who believed he was an active shooter, he shot at them to stop what he described as a grave threat.

“I didn’t do anything wrong,” Mr. Rittenhouse said, explaining why he had fired his gun eight times. “I defended myself.”

In a cross-examination that lasted several hours, the prosecution peppered Mr. Rittenhouse with questions about why he had come to downtown Kenosha in the first place — carrying a weapon and inserting himself into a highly volatile situation with a group that was unlikely to appreciate his presence.