Although this is somewhat compelling:
"2. Martin Hart
Tow-headed (the blonde/yellow king), alcoholic, and with a daughter who shows signs (see: drawings, dolls) of being introduced to sexual content at an early age, Hart blows Reggie Ledoux away rather than run the risk of his true identity becoming known. His mention in episode five of having gone hunting once is interesting specifically for his mention of having bagged a ten-point buck, the same number of points that matches Dora Lange's crown. (And, as mentioned above, he drew Cohle away from the Tuttle school that first time.)"
Not a bad explanation for why he offs LeDoux. But it raises the Q in my mind: had LeDoux seen him? That day on the property that is? If so, LeDoux likely would have said something. But I recall Hart shooting LeDoux from behind upon emerging from the house, and he was on the other side of the building when Rust first cuffed him and led him out, right?