The important thing about the Yellow King and Carcosa isn't what they signify to Reggie Ledoux. It's what they signify to us.
The more I think about it, the more I think this might be the ultimate "meaning" of the series: that at some indivisible level, life is story. Much ado has been made online about all the references on True Detective to the Yellow King and Carcosa, as if they were aspects of a coherent satanic theology to which Ledoux & Co. subscribed—a puzzle to be unraveled eventually. But it's telling that the Yellow King is a reference to The King in Yellow, an 1895 collection of horror stories by Robert W. Chambers that itself references a forbidden play called "The King in Yellow"—a play that in turn "induces despair or madness in those who read it." It's also telling that Chambers borrowed the name "Carcosa" from Ambrose Bierce, and that H.P. Lovecraft later borrowed it from Chambers.
In other words, the important thing about the Yellow King and Carcosa isn't what they signify to Reggie Ledoux. It's what they signify to us. They call attention to the story-ness of the story we're watching. They tell us, as Pizzolatto put it to me, that Dora Lange is "meant to stand in for the universal victim for this type of show"; that Ledoux, with his comically archetypal 666, pentagram, and swastika tattoos, is the universal serial killer; and that True Detective is a form of metafiction.