Khiseer Davis-Prather, 13, was with a group of friends and got off a bus when prosecutors say one of them made a gesture like a middle finger toward Tymear Johnson in March 2017.
That's when Johnson, they say, put on a mask and followed the teens to a Chinese restaurant along Hunting Park, near 20th Street, along with his buddy, Christopher Southerland.
Surveillance video shows the teen sitting with three other young friends, eating food as two men walk in. Authorities say Johnson raised a .45-caliber gun to the teen's head because he was closest and fired the fatal blow.
But the defense says it wasn't Johnson who fired the shots, that they have the wrong guy and say there are no eyewitnesses besides Southerland to corroborate the story.
The 13-year-old died after nine days on life support.
Southerland pleaded guilty to conspiracy charges as part of a plea agreement with prosecutors.