I think that the killer wanted to kill everyone in that house but, for whatever reason, he was unable to. Locked doors, got spooked, saw a dog, didn't realize the other two were present...Why show up at a crowded house, albeit sleepy and possibly intoxicated, to kill one person one has a grudge against? Too risky, when a single victim can be picked off alone at a later opportunity. There would also be the risk of armed resistance in a house full of people. Murderous grudges have time to waste, I think.