I think Eric is having a delayed reaction to all that has happened to him this past year. He has had to deal with his rape from Kristen, the public humiliation of the video shown at Brady and Kristen's wedding, Kristen escaping, his priesthood suspended.
For awhile he had hope when Daniel and Nicole were trying to find the evidence that his life as a priest would be reinstated, but then Nicole shredded the evidence and lied to him for months about it. Even when he went to Rome and could have chosen to become a priest again, all of the emotions seemed to have caught up with him and he could not handle it at that time.
Then Kristen was caught and he thought he was going to get justice for what she did, but he passed on it in order to give John a drug for another chance at life.
This is one very conflicted man who needs serious help. I don't think he's undergone a personality change in the past year, I think it is a version of PTSD, which causes him to act out, even to those whom he regards as a friend.