If you think about it, Morgana holding Gwen prisoner in the dark tower is insane for more than just yuri reasons because a large portion of her scheme depended on the idea that Gwen's husband would be the one dying to save her, but it was actually her brother who charged in first. This moment was about Gwen and Elyan, but it can also be about Morgana and Arthur, since it shows very clearly that Morgana simply doesn't recognize the power of a brother's love. To Morgana, Arthur's identity as Uther's son is far more important than his role as her brother. She cannot accept that his affection for her was ever genuine, and yet it's proved time and time again that Arthur wants to be the one catching her when she falls.
Morgana's efforts to destroy him all come down to a fundamental misunderstanding of how he feels about her. In this case, she fails to kill Arthur because she forgets that a brother will be the first to die for his sister. Even though Arthur's love for Morgana was never enough to redeem her, Elyan's love for Gwen was still able to foil her schemes.
