Who thought the professor was Bughuul?

I thought the explanation at the end would be that the professor was the one responsible for the killings all along, that he somehow possessed the kids into killing their families, and that the scary face was a mask. The first killing was in the sixties, so the professor would have been aged 20 or so at the time. He is the expert in the whole obscure cult thing. And when he is talking to Ethan Hawke for the first time we see a girl behind him (a teenager or a young woman I believe, can't remember). The girl shows up twice and in a very obvious manner, so I was thinking why show her if she has nothing to do with the plot?

I thought all the kids grew up with this psychopath and were still living with him, practicing their weird cult together, and that they were looking for a new kid to join. So I was a bit disappointed with the supernatural wrap-up at the end. Maybe they had the professor being Bughuul as an alternative ending at some point during production?