I don't see that Athelstan has betrayed the Vikings. He merely answers King Ecbert's questions and suffers from hallucinations. Now, that's not to say that he won't betray the Vikings in a future episode. I'm sure King Ecbert expects him to do just that at some opportune time in the future.
Poor Athlestan! Although I am tiring of his current state of affairs as he sits around and heals from being crucified, I do feel the poor chap is not a bum. What was he to do given his enslavement. I think he is a compassionate fellow who saw the good in Ragnar and Lagertha. He sat in the mead hall and watched Lagertha offer pity, wisdom and good governance in Ragnar's absence. (with the couple who had the baby and the husband was ready to throttle his wife for having sex with the visitor who came to their home) he saw Ragnar's love for his family and his own brand of compassionate leadership.
I think Athelstan is the kind of person who can see the good in things, nothing is black and white to him, certainly not now. He did, I think, genuinely come to care for Ragnar's band of brothers. He stopped seeing them as the barbaric heathens he was taught to fear, and started seeing them as people.
Now...why the heck he stayed behind when Ragnar left is a mystery still and has thrown doubt into the character of Athlestan.
Maybe he thought he wanted to go back to England but now that he is there, he is seeing he is not the same man he was before his capture at Lindisfarn. I think he is truly lost and confused. I agree with the post that he is probably still not sure what to make of the fact that he is actually alive.
He will be tested again, and we will see then where his loyalty is but I really think we won't see anything black and white about him again. He is split right down the middle and will remain so because that is his kind and understanding nature. He cannot wholly agree or disagree totally with the vikings or the English. He has been too intimate with both to choose a side.
My guess is, he will become a uniting force between the two sides. Given that history had the vikings assimilating into English culture, becoming guards for various kings and helping the English with other conflicts over time, we know that some how the two cultures came to accept each other in some fashion eventually. Maybe we will see Athlestan have a hand in that process.
The poor guy! He didn't ask to be captured and he just made the best of his situation. I think he has shown wisdom and incredible inner fortitude through the whole thing.