It is hard to imagine that one man alone could kill all these people. I can imagine it could be done though, if they were all asleep. If the murder weapon was a hammer, one blow would knock someone out and quiet them. After incapacitating them this way, he could go back and beat each one until he was confident they were dead. The police said two pieces of evidence link Guy, Jr. to the crime. Imo, this was his hammer (he worked construction) with DNA of the victims and his clothes with the same.
I don't think the motive was the 25,000 settlement that was won. It was being appealed so not available to Jr. at the time of the killing. Also, why not just stage a hunting, fishing accident of his father only if he wanted that money. No reason to take out the whole family. I think it was a drug induced rage that led to the killings if it was Guy, Jr. that did it. I don't put too much value in the fact that he sounded so distraught when he made the 911 call. He could have a lot of remorse hours after killing them. Maybe wished he could take it all back by then.