I would say it makes sense when it comes to plot, its the storytelling aspect of it that was terrible. It felt completely unearned as a twist. If Shelby had deranged thoughts randomly when you played as him, or some sort of darker side was shown while playing as him, the twist would feel more earnest. Instead, either by coincidence or luck, he never has inner thoughts about what murdering he does while not being a weird scumbag (gameplay sections where you play as him).
The twist is so unearned that any of the playable characters besides Ethan could've been the killer. The future cop dude or Madison Paige could've been the killers and not much would've been different.