I think its Dr Royston Daniel from Season 1 Episode 18. I think its possible he has never actually killed anyone himself but has built up a network of 'agents' through his hypnosis skills thereby providing himself with the perfect alibi.

I also think the whole 'Visualize' thing is just an enormous red herring (pun intended). 

The episode where he appears is quite clever in that it makes him seem weak - he becomes the victim (in our eyes) and is nearly murdered until Patrick Jane saves him but I think that was all an elaborate ruse to get close to the CBI team - I think someone on the team was hypnotized and has been feeding RJ information - probably Van Pelt.