Red John is a fan of women with red hair, a fact made very clear from the beginning . Van pelt has red hair and all the 7 (6?) suspects have had contact with her. For continuity and realism it would be safe to say that Red John would pay her at least a little bit of attention. 
Sheriff McAlister is the only suspect on that list i remember paying her some attention. 
Also did anyone notice that the sheriff insisted that PJ call him by his first name? something red john likes to do i.e always calls PJ by his first name. 

Im not convinced he is red john though as evidence seems to be been pushed down our throats in the last 2 episodes.