"Anaheim police said a 13-year-old boy allegedly threatened to shoot the officer, who then attempted to detain the boy until police arrived, sparking a physical confrontation. But the boy's mother disputes that -- saying her son said he was going to sue, not shoot, the officer."
"Two teens were arrested -- one on charges of criminal threats and battery and another for assault and battery, police said"
Assuming the charges hold up, and with the pack of feral youths surrounding and even assaulting the off duty office, can't really see where he erred discharging his weapon. Also the video's I've seen only start when the cop is dragging the kid away, nothing beforehand (if there is one please link) which I'm going to guess is more telling then just what was seen.