We are talking about a young man that has the following difficulties
he is out of shape
he has a major head injury
he has schizophrenia (could be treated and not a hindrance)
At no point has anyone stepped forward and said that this young man was intellligent. If you look at the characteristics of someone with a traumatic brain injury, they tend to be impulsive, explosive and scattered.
I do not believe in my heart of hearts that this man could have done this alone. Was this his idea? Maybe? but he got help, whether it be after the fact or it was planned before.
He would have had to container all four men (could be at different times), kill them, transfer their bodies, dig the hole, dump their bodies, fill the hole, move all the cement, hide the car (didn't do a great job of this and left one of the victims have their cell at the burial site - PING). His skills with the car (messy, messy job of being a criminal) do not allow me a lot of confidence in his ability to get this job done.
There is a lot more to this story, IMO than one boy killing 4 boys and burying them.
You are right... it defies logic.