That's how US law works, though. Guns are legal through the Second Amendment, as is their usage in legitimate self-defence.
Crimes against the person, such as mugging and burglary, are considered much more serious than they are in soft Britain. The burglars had already inflicted a serious, though admittedly not life-threatening, injury to the victim, and any court in the land would I'm sure adjudge that he genuinely feared for his life and was perfectly entitled to use deadly force to protect himself.
As stated above, the young man should consider himself lucky that he at least is still alive.