In the preview, I'm also thinking when Marty is with Rust looking at his evidence and screams "No"...it's something to do with Marty's youngest daughter. The crown getting stuck in the tree a couple episodes back leads me to believe she's going to play some role in this.
It looked like he was watching video in the preview. I think it's footage of Maggie seducing Cohle. The way he reacts it seems like he's shocked/disgusted, but not surprised by it. Based on the way they've been playing the preview clips out of sequence, I think Cohle shows him the video when he first gets to his 'workshop/storage bldg' to get him on board and show him that the 'affair' didn't play out the way Maggie made it seem, because I'm sure Marty is still holding that betrayal over his head.
And I don't think the 'Tall Man' and the 'Man with the Scars' are the same guy. I think the Tall Man is Tuttle. The director may have just been toying with the audience with this, but the way he framed the shots of Tuttle looming over Cohle at the church offices, exaggerating the height difference, made me think he's the Tall Man. I think maybe Cohle did poison him, and then when the killings didn't stop, figured out things go much further than even Tuttle.