The guy was outnumbered, so clearly he needed to take the first shot.
On a serious note, though, we don't know enough about Sailors to make any judgments. Did he have any sort of mental problems or history of violence that would make him a high risk gun owner, let alone a legal one?
And how would this have been prevented? Let's say he was a legal, law-abiding gun owner who was overreacting to paranoid rhetoric. How do you regulate paranoid rhetoric? How do identify who will overreact to it? How do you even define what it is? Home invasion does occur. Homeowners are injured and killed in the process, albeit rarely. What is a sensible level of warning, and an appropriate level of caution?