The guy who broke into the house committed a criminal act -- breaking and entering. The guy IN the house has a right to defend his castle. The law does not require him (nor, IMHO, should it require him) to be an expert marksman in order to be allowed to defend his castle. The responsibility and the liability should not lie with the guy defending the castle, but with the guy invading the castle.
Let's just say that we agree that a person has a right to defend themselves and their loved ones, but we do not agree that a homeowner should have be permitted to execute an intruder. I also noticed that not one person had a word to say regarding immunity where the homeowner greviously injures and perhaps maimes or kills a perfectly innocent bystander.
Under your theory a homeoner should be immune from liability, criminal or civil, for shooting a drunk who foolishly confused the homeowner's home with the inebriates, even if the homeoner shoots them as they lay peacefully passed out, for all purposes dead to the world, and totally helpless.
Do you honestly think that the ioccupant of a dwellilng should have the right to take the drunks life? Assume that the premises are well lit, the homeowner knew that the intgruder was passed out, and the homeowner recognized the intruder as his friend and neighbor, Charlie, who in the past has made the same mistake and passed out on the homeowner's couch thinking it was Charlie's. Add to the hypo the fact that the homeowner and the other three occupants of his house are expert sniper instructors assigned to a local Navy base and each one is awake and armed with conventional small firearms plus cans of mace.