I'm a believer that there were at least two people.
A single killer, I could see, if it were all in one home, yes. I just don't see it in this case. I believe there would at the very least, have to be one other person assisting. I could be wrong. It's happened before.
Fratricide is as old as Cain and Able but, there's at least one sibling, in this scenario, who I just don't think has it in him, nor had a reason, to commit murder.
I personally don't see "common sense" at play in any murder. The assailants, however, have their justifications. Whether it is revenge, saving their own hide, greed, envy, anger, etc... However, it is different. When it was released on the networks, I remember them saying that nothing quite like this had been in the news in decades. I've not been able to find another family annihilation quite like it, nor one where the assailant(s) didn't either commit suicide, or were not caught shortly thereafter. I've not found one where someone has gone down a road, entered four homes, all of relatives, and murdered everyone in each home, and disappeared into the night. The location of the homes helped them to do so, but they apparently left little evidence or they'd have been caught by now. Four homes. Eight people. All owners of firearms, I'm guessing, and most likely adept with them. I'm no idiot, but, I'm sure I'd have left SOMETHING behind. Unless, I either already knew how to pull it off, w/o leaving evidence, or I had planned this for weeks, if not months. I mean meticulously planned, and researched how to pull it off. This was no spur of the moment thing. The average Joe Blow could not just walk in there and do this w/o leaving evidence, unless they were smart enough to know what to do, or research what to do, to get by with it. I'm honestly very surprised that an arrest wasn't made within the first couple of weeks.