You know, a friend of mine was on a jury for a murder case a little while back. Two teenagers broke into some old guy's house to steal some cash after seeing him use an ATM. They tied and gagged him with his bedsheets before taking the money and going, only they tied/gagged him too hard and he suffocated. Sure, they didn't mean to hurt him, and they were only after a couple hundred bucks, but the result of their actions was that man losing his life, and receiving a conviction of murder 1.
This case isn't that different...the biggest difference is that the man had a gun to protect himself. If he was telling that kid to stay down at gunpoint, and the kid was stupid enough to get up, he deserved it. I can picture the scene, with the freaked out old man cornering 3 equally freaked out kids, when all of a sudden one of them decides that his balls are too big to do what he's told, gets up to mouth off, and catches it. The fact is, things could have gotten really ugly really quickly had he not pulled the trigger. Let's just say that the kid managed to trip him up/disarm him before they all escape, and in that trip he falls and breaks his hip; that's a very serious, traumatic injury that could eventually prove fatal, and that's from something as simple as pushing him over...let's not even get into what could have happened if those three tried to beat him while he was down.
I look at it this way...they screwed up the second they decided it would be a good idea to do a B & E for refreshments. Then after getting caught, they screwed up again by not listening to the man with the shotgun. I don't know what I would have done in his situation, but I know if I felt like my life was in danger, I wouldn't regret doing what I felt had to be done to protect myself, which is something I don't think anyone should have to apologize for.