Even if the kid's whipping your ass you have no right to shoot him. The only way it could even be close to justified is if the guy actually feared that the kid was about to actually beat him to death, which I doubt, or, if the kid grabbed your gun, you struggled, and the gun went off.
If you leave the house with your gun you're the one that needs to make sure retarded incidents don't happen.
Call the cops and let the cops handle it. That's it.