Arthur's heart felt heavy, if such a thing was possible. The feeling was not entirely foreign; over the years, he had been required by his father's directives to do things with which he disagreed, and they weighed on him. Still, this was a different heaviness.
When Merlin had recounted some of the events of his point in time, they had sounded incredible, but not completely impossible to grasp; however, he had omitted Morgana's part. Guinevere's version of events was markedly different, in that it was not only Morgause rampaging across Camelot: Morgana was right beside her, the whole way. And, she had done things which Arthur could not fathom, in the process. He could figure why Merlin would neglect to mention that, but was glad that Gwen had filled in the gaps of the story.
Hopefully, they could change things, now that they had this information.
As he walked into Morgana's room, he wondered how he could possibly tell her those things. How did you inform someone, especially someone dear to you, that they might become a monster? The last thing he wanted to do was to hurt her, but she deserved to know, as much as anyone.
Nodding in response to her question, he slowly took a seat, still trying to figure out what to say. "It is a difficult subject to broach, and I can think of no way to do so gently." Reluctantly meeting her gaze, he said, "What Merlin told us of the future, about Morgause taking Camelot... he left out the fact that you were allied with her. That is why you took the throne." And imprisoned our father, and hunted me... why you killed innocents, though, I do not know. None of that needed to be said, he hoped, but it was in his mind.
He dropped his gaze back down. "Guinevere told me this, in the hopes that we might be able to prevent it. And that is something I certainly intend to do, as I am sure you do, as well."