I can see why he shot the BG a second time. This is the BG that not only made threats against them, not only went out of his way to bang on the grandparents' door and make more threats, but held a knife on the girlfriend while threatening to kill them both. He certainly did a good job of convincing them that, so long as he lived, they were in danger. If the first shot didn't kill him, he could have come back after his hospital/prison stay, and tried again. No way to know if he really would have done that, but he sure convinced the young couple he was capable of such. I can't say I wouldn't have fired the second shot, either (can anyone who has family predict what they would do in this young man's situation? probably not).
Granted, all this means nothing unless/until it's presented to a jury, and I have absolutely no idea whether they would convict. I'm sure this WILL be brought before a jury, though.
TXInvestigator, I have to say you're unfortunately most likely correct. I can understand why he fired the second shot, and don't necessarily disagree with the necessity of that shot; but this young man is in for a world of legal hurt.