A part of me is able to understand that rationale or lack thereof; however, another part of me thinks if his father was the catalyst to the rest of the family being murdered, then would he have loaded his weapon to capacity? Clearly the rifle was loaded to capacity from his retrieving it from his room to standing at his parents doorway because of the rapid succession of the murders. Say he loaded the weapon with but two bullets, shot his father and then the rest of the family, at the very least his mother, reacts to the gunfire, if his intention was impulsive and directed solely at his father, then the rest of the family, again, at least his mother, wouldn't have been found just waking up or rising from the bed before he decided to take the rest of them out by reloading the rifle and then shooting everyone else. I feel he was aware enough to have given some forethought to his initial action and maybe that sense of question came after ending everyone's life in the house. Keep in mind, at the time the murders took place, Shaggy was already outside of the house. This, too, is indication of intent, a formula, if you will, as to not have Shaggy somehow impede his actions that evening. To what degree, I am not sure. Yet for whatever that variable was, he secured the dog elsewhere, would have to at least return to the second landing and move swiftly...again, evidenced by the apparent lack of more overt reactions of his brothers and sisters.
For the record, I can see his shooting his mother in reaction to him shooting his father. That would support an almost instantaneous response, but the others, he would have had to turn around and walk to three separate bedrooms with each step in their direction an intended one.
"Everywhere I went wuz like uh telephone; no answer."