"What If" he didn't have a shotgun, and the thieves saw him, and didn't want to be identified, so they decided to kill Mr. Horn with the crowbar?
Either way he did what he felt needed to be done, and the courts ruled that what he did was within the law.
What's freaking so many people out i think, is that it was caught on tape, and maybe they mentally can't handle it. Maybe in that same situation alot of people wouldn't have even called the cops in the first place.