I've got a few notes on that:
1. There's actually one plot point that was mentioned earlier, but I'm not sure whether it's actually relevant or if the dev just forgot about it. Back at the end of Act 1, we're told that
2. During the investigation at the farm, we're told that the smell of the creature is "all mixed up", suggesting that the answer might not be as straightforward as naming a single person. Especially since whoever it was seems to have completely changed priorities and suddenly run back for no reason, possibly because there's more than one personality at play - and possibly more than one form.
3. But then there's a more straightforward, classic detective cliche.