The 30-year-old Good Samaritan was trying to help a robbery victim who was being beaten up in a parking lot of a strip mall on the 18000 block of Soledad Canyon Road, according to authorities.
"I went out and saw a guy went down, and I thought that maybe he was just out of conscious, so we all tried to help," said witness Chris Roongfamgarm. "By the time we got there, we knew that he couldn't make it."
Detectives said a 29-year-old man had arranged to meet the four suspects to sell a Nintendo DS game system.
Instead, the suspects attempted to rob the man and a fight broke out. That's when the Good Samaritan rushed in, armed with a baseball bat. According to investigators, one of the suspects shot the man in the torso.
"It's very tragic, actually, because we have somebody who is intervening, trying to do the right thing, trying to help somebody out," said Lt. Eddie Hernandez of the Los Angeles County Sheriff's Department.
The gunman also shot the robbery victim in the leg as he tried to escape. He was treated at a hospital and released Tuesday.
The suspects ran from the scene, but all four - three juveniles and an adult - were caught a short time later.