Hi everyone.
I re-watched the episode where that crazy Rebbeca  woman kills Bosco's team. When they are taking her away, the man coming towards her poisons her (as we know).
If this man is Red John, then it can only be Brett Partridge. They show him for a second from the back and he is tall and thin and has BROWN, short hair (definitely not white, grey, or bald, but dark brown hair).
Of course it could be someone who just works for Red John. However when this person is coming towards crazy Rebbeca, she has the happiest, most joyful face, like she is seeing God or something. (please watch it if you have time and you will know what I mean). This makes me think that it was RJ personally, who poisoned her. But if it really was him, then it can only be Partridge from the list. He is the only tall(ish), thin person there with brown hair.