It's Bob Kirkland.
Either HE is Red John or he is an assistant and I'll tell you why.

1. He shook hands with Jane. Jane said "Do I know you?" and Bob said, "No, but I know you."
2. He killed the guy at the hospital who knew Red John and said, "Do you know me?" or something along those lines.
3. ffs he broke into Jane's house and looked at his board of clues to RJ that's how he knew the 7 suspects.