The problem with autism is that it's impossible to predict how a kid will act/react. A close friend of my mother's was director of a school for autistic kids for many years. Every once in awhile she come for a visit with bruises or bite marks. Mostly autistic kids are ultra introverted, but ... yeah, they can have "anger management" issues.
Nonetheless, even knowing (second-hand) something about autistic kids, I don't think it's reasonable to claim that the father "should have known" his kid would get so angry/vindictive that he'd break a window to get into a locked room for access to a rifle. That sort of anger/pre-meditation mix isn't typically autistic. That's bordering on sociopathic ... if it's not actually over the line.
So far, I don't see any blame on the father, but I would like to see some follow-up reports.