I first thought it was Ray Hafner but then I re-watched S6E2 and saw a a few things that made me think it was Reede Smith.

1) In lisbons dream he is holding the knife in his left hand.
2) In the scene with Sheriff Mccalister and Bertram in Bertrams office - when Jane leaves the room Smith says to Bertram something(I forgot what he said) but it sounded like he was talking with authority and Bertram answers to him.  Also Mccalister didnt say a word.  
3) When Jane left the office he whistled (I know this is weak but thought maybe its a clue)

This isnt evidence but a thought - in the last episode of last season when the suspects are revealed and the pictures are placed ono the desk his is the one in the middle.   I think maybe the suspects surrounding him are the closest accomplices/disciples etc.  Partridge was at the right end.  Id be curious to see if Stiles(whose picture was at the left end) is not a major accomplice

I havent followed as carefully as others so maybe someone can help w evidence of my theory of Reede smith