As above, cannot see it as at all contrived and certainly not predictable. I think it's more of a disappointment on some people's part that it wasn't a twist and the killer wasn't a high profile character or some grimly machiavellian shadow of a man sat behind a large desk in a mansion etc.
***SPOILER*** (yeah, where's the code?)
The killer was as a product of generations of abuse himself and there was some talk of his having been groomed for some form of transcendence. Anyway, we're talking about many people here that were committing these crimes. These shitkickers had a ring and rings are based around everyone being in key positions to keep the ring closed. The killer happened to be the caretaker for one of their ritualistic sites and apart from his appearance and general redneckishness, by virtue of the way he lured the detectives into Carcosa and stalked Rust, he didn't appear to be an idiot at all. Quite the opposite as he affected different personas when speaking to different people, even putting on that educated trans-atlantic accent while watching TV. The showmakers were showing us that this guy, while matching our idea of how a redneck simpleton would look, was not an idiot. A vicious primal beast of a man, but not an idiot.