I think his only legal hurdle is that he didn't have the gun on him at the time he found the man in his daughter's room. Pausing, even very briefly, to get the gun from his wife probably constitutes "premeditation" in the courts.
Reminded me of a sad story from a while back. A dad and his two sons were changing a tire outside of their house. A drunk guy sped around the corner and hit the truck they were working on. Both kids died instantly, but the dad didn't get hit. He went in his house, grabbed a gun, and shot the drunk driver to death. I can't even imagine the sheer rage and agony the man went through in those few brief moments. Problem is, he was charged (and I think eventually convicted) of murder. Despite his emotional state, having the mental wherewithal to go in his house, open the safe, and grab the gun constitutes "premeditation". If he had the gun on him, he probably wouldn't have been charged at all. If he beat the driver to death, same thing.
But taking the time to go get a gun from the house is the fine line that he crossed. Sad situation all around.