Spoilers from here on in.
I love to guess the ending of a story, especially for a mystery. Though most times I find myself recognizing the story beats in television well enough to guess the ending, True Detective still feels wide open. The depth and weirdness of True Detective's mythology has inspired a lot of internet speculation about what's really going on and how it's all going to end. "After You've Gone," the penultimate episode of True Detective, exploded a lot of those theories. You can find some great thematic & theoretical breakdowns here, here, and here.
"Hart and Cohle have not been secretly working together on the case." That is, unless you believe that they've become truly self-aware, know they're on an HBO series, and are deliberately tricking us. That might be a little too weird.
"Hart and/or Cohle know what's going on or are behind it." It's fair to say that, based on "After You've Gone," that neither of the two leads are the Yellow King. That is, unless one of them is the Yellow King and doesn't know it. Or, hasn't become the Yellow King yet, but will in the "future" before becoming unstuck in time a la a Kurt Vonnegut novel.
"Gilbough and/or Papania know what's going on or are behind it." These two are even more clueless than Hart and Cohle in "After You've Gone." Or they represent the repetition of the cycle, ignoring the creepy Childress lawnmower man just like Hart and Cohle left another at the Light of Day.
What's still possible:
Occult conspiracy, cthulu mythos, creepy Texas Chainsaw Childress Family values, one of Hart's daughters is the killer, Maggie masterminded everything and broke up Hart and Cohle to cover her tracks, the idea that causality is a lie based on our perception of time and therefore no one has to kill the victims for them to die in that manner, pretty much anyone but the detectives could be and probably is the Yellow King, time travel, OJ Simpson, Canada, aliens, drugs, just some random crazy person did it.
But let's be realistic:
It's clear there's a child-abusing/murdering cult. But it's unrealistic that a cult trying to keep itself hidden would be displaying ritualistically murdered women for the police to find. So we're probably looking at to big bads to take down: the cult and the murderer. The murderer is most likely familiar with the cult, but not actively a part of it. Best guess is our friendly neighborhood scar-faced tractor-driver Childress, probably kicked out of the cult for being lame and killing women in an attempt to recreate the cult's rituals.
Does it really matter how it ends?
Not really. Unlike shows like Lost which seem to hinge on a future payoff, True Detective has been so awesome from episode one that the end really doesn't matter. Though I can't wait for it.