For anyone who didn't understand the plot twist :::spoilers:::

Obviously, the killer was the boy from the beginning. He was showing signs of psychotic tendencies so his parents chased him through the snow and buried him alive. He didn't die and returned to the lodge where he attempted (and possibly succeeded) in burning his parents to death while they slept. This results in the lodge shutting down since the parents were the owners.

The sequel also delves into the back story a little. A sheriff is told but a man who knew the boy's family that when the boy was born he was a stillborn. But hours later he suddenly started breathing on his own and, as he grew up, he began showing signs of being deeply disturbed.