Perhaps, but I'm still in the camp that he did it. So in that, him "turning" might not be a turn at all. He's smart as hell, has a past that is getting pealed back slowly and if you want to say that he was too wuick to be a brute in prison then you can also say he had it in him already and it was part of the plan. Of course, just wild speculation on my part.
But I'm still leaning towads he did it but has this entire persona that makes him look innocent, Stone sacrifices himself for him thinking he was innocent to the end and in the final scenes we find out that not only was he guilty but he might just be a long time bad guy.