So, a few thoughts came together in my brain and sort of clicked into a heartbreaking realization.
Chris is Josh’s best friend.
Josh used to talk to Sam after he lost his sisters, and said she was one of the only ones who understood him; she thought they had a bond. Sam also, having been the twins’ best friend, would be one of the last living non-familial links to his sisters.
Chris and Sam had nothing to do with the prank on Hannah, Chris being passed out and Sam having actively tried to stop it.
These things probably mean Chris and Sam were the people Josh cared about most.
Dr. Hill says "Why did you hurt them? It’s yourself that you despise.“ and says various things that make it crystal clear that Josh blames himself for his sisters’ deaths (’couldn’t lift a finger to help them’, etc etc).
Josh specifically tortures Chris and Sam (and Ashley, but she seems more of a tool with which to torture Chris than an active target of the psychological torture), the two who were least involved in the prank, and then claims it’s revenge.
The heartbreaking realization:
It’s not revenge on them, like he claims: it’s revenge on himself. He’s hurting the two people who mean the most to him, the two people who it would hurt him the most to see hurt or upset. His ‘punishment’ of his friends is just a self-punishment, inflicted by watching the people he loves most suffer and knowing he is the cause. He’s trying to fool himself most of all when he says that it’s their punishment and they deserve it.