This one of those cases where people start talking past each other and get stuck on the minutia of semantics. The poster that says gun culture explains this is right, the posters who say the home owner had a legitimate right to shoot the intruder are also right.
The reason the home owner had a gun is because of a culture of guns where he knows that there are dangerous people out there who can break into your home and do you harm with guns. Therefore this culture gives him legitimate reason and right to get a gun to protect his home. Because of our gun culture he had the reason to believe that the person that was breaking down door after door to get to him probably had a gun, so he responded based on the culture in which he finds himself.
If this was a gun-less culture where people resolved issues without guns, the home owner would not have the need for a gun because he would not be in fear of intruders with guns. Maybe he would have lost a fist fight or got killed by a lamp slammed on his head. Someone would probably have got hurt from this incident. Nothing good happens when an intruder breaks down two doors to get to you.
I really do not see how you can blame the guy who shot in defense of his castle. If you cannot be safe in your own home, where can you be safe. The response here was proportional to the realities of our gun culture.