Too many "unknowns" here. The guy's age being a big one. I took care of my Dad for the last 10 years of his life. By the time he was 85 his brain was in a sorry state...as confirmed by scans.
Also,I've read that the kid actually *entered* the house. If that,by chance,is true that works in the shooters favor.
I'm completely willing to believe that this kid meant no harm...that he just made a silly mistake. But,OTOH,I've seen no evidence to suggest that the shooter was evil. I'd be very willing to believe that the shooter was just a scared old guy.
If I am the lawyer for the old man, I will instantly claim my elderly distraught client did not understand his right to remain silent.