The way I read it there was a struggle over the gun and it fired. Cop probably was twisting the gun in the teens hand and it fired during the process. At least that's the way I read it. If so, the threat was continuing as long as the teen's hand was on the gun.
Frankly, I think this event turned out just the way it should have. One down and how many thousands more to go?
Chicago cops certainly have a bad reputation but I can't find much fault here based solely on the report.