Thomas Autry, who authorities said will not be charged, had been walking home from his job waiting tables Monday night when four people got out of a car and chased him, Atlanta police detective Danny Stephens said.
One of the attackers had a shotgun and another had a pistol, Stephens said.
The suspects caught up with Autry, who yelled for help and pulled a knife out of his backpack. He kicked the shotgun out of one of the attacker's hands and stabbed both a 17-year-old girl who jumped on him and a man who also attacked him.
The suspects fled in their car but police found them later at a hospital where the girl was pronounced dead. The man stabbed in the incident was in critical condition, Stephens said.
Autry's attackers will face robbery and aggravated assault charges and are suspected in other robberies over the past week, Stephen said.
Stephens credited Autry's military training with helping him fend off the group.
"I would say he had to do what he had to do to stop the threat," Stephens said. "You can tell his training kicked in and he knew what to do."
Autry, 36, suffered a cut to his hand and a bruise on his chest, Stephens said.