'Mare of Easttown': New Killer Theory Pins Murder on Overlooked Character

This article contains spoilers for Mare of Easttown's sixth episode, "Sore Must Be the Storm."

HBO's murder mystery Mare of Easttown has fans obsessively theorizing over who murdered Erin McMenamin (Cailee Spaeny). The limited series starring Kate Winslet has offered no easy answers in its first six episodes, and things certainly aren't clear going into Sunday's series finale. While Detective Mare Sheehan (Winslet) believes that Billy Ross (Robbie Tann) is both the father of his niece Erin's baby and the man who murdered her, whatever is on the photograph that Jess Riley (Ruby Cruz) showed to Chief Carter (John Douglas Thompson) seemed to throw major doubt on this assumption.

There are many threads that have to be tied off in the finale episode, and it will be fascinating to see how correct the many fan theories are. The leading theory is that John Ross (Joe Tippett) is the actual father and killer -- the pendant's receipt only indicated that the buyer's last name was Ross! -- making the infidelity allegations even more sinister than expected. Vanity Fair's Joanna Robinson also put forth a theory that may seem a bit out of left field at first but makes quite a bit of sense when you look at some of the smaller details from the previous episodes. Robinson poses an interesting question: what if Ryan Ross (Cameron Mann), John and Lori's (Juliane Nicholson) son, is the killer?

Viewers saw John telling Ryan to keep a secret just between the two of them, which Lori saw and assumed that John was cheating on her again. However, it could be even darker than that. Ryan could have known that John was involved with Erin and killed her in a rage for threatening his family. The young boy has been prone to extreme melancholy and violent outbursts all season, so whatever he knows is obviously taking a toll. Now, this isn't an airtight theory, because it seems unlikely that a young boy could be out in the woods so far from home at 2 a.m., but it's not impossible.

Robinson also points out that if John is in fact guilty and also manages to both pin the murder on Billy and kill him in order to silence him, it would be poignant for Ryan to be the one who exposes his father's guilt. For Mare to successfully connect with one troubled boy after she was unable to save her own son would be a fitting conclusion for a heartbreaking series. Sunday certainly seems impossibly far away.