There is no way in hell that the Karma Houdini aspect of the ending will hold up. Mackey is a well-known former cop, a notorious Serial Killer whose identity is known and his crimes 100 percent proven, completely isolated from any avenue of escape. There is going to be the largest federal and state manhunt in history for him. His freedom and threat to Davey aren't going to last more than a few weeks if he's very, very lucky. While Davey might be haunted and traumatized by the Summer, he won't be living under the threat of Mackey's return for very long. In support of this, as mentioned on the main page, very few serial killers have managed to escape justice after being identified. Not to mention that Mackey was also a Corrupt Cop... the Oregon police and FBI aren't just going to take Mackey's crimes seriously, they're going to take it personally. Looking at the ending from another perspective, the point wasn't that the killer will make good on his promise but to make Davey spend the rest of his life worrying if Wayne will collect on the threat.
Woody's mother was already suffering signs of depression when we first saw her, so it wouldn't be too far-fetched that the murder of her only child could drive her to suicide.
