Who was the other killer? Over the course of the film, we hear quite a bit about the Memorial Day Massacre, and even learn that one of the perpetrators of it was the villain's father:
One problem, though - what happened to the other killer? We know that there were two killers because, in the villain's flashback, they are clearly visible:
Now, one might say that this is the consequence of his status as an unreliable narrator, but since the story came from his granparents, who presumably heard it from his institutionalized mother, it's pretty trustworthy. Beyond that, consider Tucker's cabin-
It was clearly the property of the Memorial Day Massacrer, and since the cops caught one of them, but never found the cabin and all the evidence inside, it must have been the property of the second killer.
So Tucker bought the cabin from the other killer. Will this, combined with the survival of this film's villain, provide the foundation for a potential sequel's plot? We can only hope!