As noted on reddit, Hart is actually an Old English word meaning “stag,” potentially linking him to the antlers that crowned the murdered Dory—and perhaps signifying his own crown. After all, if we're looking for a yellow king, Martin's the blondest guy around. When Cohle made his way towards the Tuttle school the first time, it was Martin who drew him away by honking the horn, delaying his discovery of the twig sculpture and the possible darker truth behind it all. When you consider the possibility of Marty as the Yellow King, suddenly it seems awfully convenient that Marty killed Ledoux in a fit of rage, ensuring that no one would live to give him up.
In general, Marty doesn't treat women and children that well: He cheats repeatedly on his wife—once with a former child prostitute—and gets violent with Maggie after he learns of her infidelity (not to mention slapping his daughter Audrey and calling her a slu*t). We've also seen Audrey making sexual drawings and arranging her dolls in a sexual way at a young age, as well as her later alienation and promiscuity in adolescence. What if she's been sexually abused by her father, or by someone else with the approval of her father? We've been looking for monsters in the shadows, or perhaps buried in the complicated psychological labyrinth that is Rust Cohle, but what if it the real monster was in front of us the whole time wearing the simplest mask of all?
The Lawnmower Guy is the Yellow King
This one feels a bit more anticlimatic than Rust being the King—a bayou version of "the butler did it"—but the semi-regular appearances of Errol the lawnmower guy give him enough of a presence to feel like an unexpected culprit without coming completely out of left field. Sure, he seems like a simpleton, but what if he's much more? With his green clothes, scraggly hair and beard, he also looks a bit like the “Spaghetti Monster” the young girl said chased her through the woods.
On a symbolic level, one of the earliest fights between Rust and Marty—and the foreshadowing of Maggie's infidelity—was Rust mowing Marty's lawn when he wasn't there. If lawnmowing is linked to sexual transgressions, what could that means about a man who devotes his life to it? And what lies under that beard, anyway? Could it be the scars that Dora's friend said were on the face of the man who visited her before her death, the same ones the rescued girl told Cohle was on the very worst of her abusers?
Governor Tuttle Is Actually the Yellow King
It's easy to see why Governor Tuttle, a high-ranking government official, might be in charge of the cult. If there really is a police cover-up of the murders and child abuse, that would require a lot of power, which Tuttle certainly has. His family connections offer more links to the cult: His cousin, the late Reverend Billy Lee Tuttle, seems to have been behind the funding of the Wellspring funding of religious schools, which have been linked to the ritual abuse. We haven't seen the shadowy Governor Tuttle yet, so it'd be a bit surprising to uncover the identity of the King only to learn it was some guy who's never been onscreen. But who can say?
Maggie's Dad Is Actually the Yellow King
Audrey's in-no-way-traumatized playtime with her dolls
A variant on the Marty theory, this suggests that his father-in-law was the familial abuser who molested Audrey instead. This trauma produced not only her sexual drawings as a young child but also the sexual configurations of her dolls that mirrored the abuse of the cult. There's also that moment when Audrey takes a princess crown—linked to the crown symbolism around the King—and throws it up in the tree where her sister can't reach. Is that a symbolic way of protecting her from the abuse?
