He was known as the Mayor of 95th Street, an angular figure in a cap and blazer who was a fixture of the streets of his Chicago neighborhood. He was an activist who tried to help young people rise above the city’s unrelenting gun violence.

But William Cooper, 58, fell victim to the violence he had worked so hard to stop.

On Monday, Chicago emerged from another bloody weekend of gun violence in which 10 people were killed in 35 gunfire incidents from Friday evening through just before midnight on Sunday, the Chicago Police Department said on Monday. Among them, the department confirmed, was Mr. Cooper, known as Willie.

Outside the Lilydale Outreach Workers for a Better Community in the city’s South Side neighborhood, local television stations broadcast an impromptu prayer vigil and a shrine that marked a spot at the nonprofit where Mr. Cooper had counseled the city’s young people.

“People are so coldhearted,” Mr. Cooper’s niece, Patricia Carter, told WLS-TV. “How could you take somebody’s life? He helped everybody. I just don’t understand.”

While the city’s gun violence has often left families and neighborhoods reeling, the weekend tally of victims hit particularly hard when it came to light that Mr. Cooper, who had made it his life’s work to help young people find…