Warning: Spoilers for True Detective
Happy Premise #1: Time is a flat circle. Everything we do in life, we will do over and over again, with no memory of our past and no vision of our future.
Happy Premise #2: This means that whatever we've experienced in our lives, the joy and the suffering, will be ours for eternity. There is no escape. Murder victims wake up into the same nightmare again and again.
Happy Premise #3: Consciousness, therefore, is a tragic misstep. The rational thing to do would be for the entire race to commit suicide and deny our programming. But we cannot, because we did not therefore we will not. We are carts on a track who despise the track but cannot jump off of it.
Happy Premise #4: Therefore the most blessed salvation is to die pure, innocent and with a minimum of pain or fear, like Rust's daughter. Just a short, sharp shock and then we slip into unconsciousness, never aware that we died. And so we don't wake up into a nightmare. We don't wake up having to end up in Reggie Ledoux's party box again and again without even knowing it until it is too late. One day our life just...stops.
Happy Premise #5: Once you become aware of this fourth dimensional perspective and view it from the outside, as a circle and not a sphere, you immediately become capable of breaking your programming. You see life as it is and you see your best path is to be a willing sacrifice to this message. To martyr yourself as a symbol of the path to salvation. That is the riddle of Gesthemane, allowing your own crucifixion. That is what is meant on the cross by "it is accomplished" (not from a Christian perspective, but a Carcosan perspective.)
Happy Premise #6: Reggie Ledoux's psychedelic mixtures provide you with this perspective.
Happy Premise #7: The person killing children is not murdering them in his mind. He is opening their eyes and allowing them to choose to spend eternity as (relatively) innocent children/young adults. The longer they live in the thresher, the more horror they will see, the more trapped they will be.
Happy Premise #8: Cohle agrees with this completely, but he cannot accept it because his programming is that he is a homicide detective and drugging people into being willing victims of sacrifice is a terrible crime, not an act of salvation. He has been persuaded by the logic of the Yellow King, but cannot break out of the course he's set for himself to take the King down. And now he is trapped in eternity, forever resisting the one great cosmic truth of the universe, the truth he cannot mainline because he cannot find a vein.