Still he did move on and vowed to save the underworld and give hope to all when he understood the prophecy now. Remember?
I….don’t think that means everything he did in his past no longer existed. He still did all those evil things of his own choice. Helping Hook to find a book because he feared the consequences of those evil actions (not out of genuine remorse and regret mind you) doesn’t suddenly make him a hero. If Arthur had expressed some kind of feeling that he knew what he did was wrong and was on this journey to fix it all, i’d buy it. But he just doesn’t want to go to Hell. It’s not sincere. Not to me. Now if I were the writers, it wouldn’t be that hard to have Arthur return as a villain. Arthur desperately wants a kingdom. When Camelot failed, it was SB. When that failed he is set on the underworld. He could very well be down there enslaving everyone somehow. I am saying its a possibility. I personally didn’t buy that the portal was opening for him or that he was committed to “Fixing” the broken kingdom of the underworld. It just doesn’t make sense to me personally. Just seems Arthurs really wants to be a king.
Suit yourself. But also remember this. He helped the Charming’s and Grumpy to save Regina and Robin from the fury. That didn’t sound like a villain’s work.