It absolutely does not justify that. You are not suddenly immune from the damage you cause because you were defending yourself. If you don’t pay attention to what’s down range and put a bullet through a kid’s head, your negligence caused a death and you will absolutely be charged with manslaughter at the very least. Most people don’t realize how loud firearms really are. Discharging a firearm inside an enclosed space without hearing protection can permanently damage hearing, so he could have easily permanently injured a lot of bystanders.