CHICAGO — He was part of a group trying to stop the gun violence that has plagued this city. But this week, Delmonte Johnson was killed by it.

Mr. Johnson, 19, who was fatally shot along a South Side street on Wednesday evening, was part of a group called GoodKids MadCity, which emerged in Chicago following the deadly school shooting in Parkland, Fla.

The group has focused on curbing gun violence by pushing to create more jobs for young people and improving mental health treatment. Mr. Johnson was often among the first to show up for events and meetings, organizers said.

“I’m not surprised that something like this has taken place, because unfortunately, he’s one of many people that this has happened to here,” said Carlil Pittman, 25, a leader in GoodKids MadCity. “But to think that a young person who was out there trying to fight this issue was taken down by this issue is really heartbreaking.”