After all the Merlin fanfics I have now read, I’m genuinely surprised that none of them ever considers that Morgana didn’t turn evil naturally. It literally says that her bitterness spreads like a disease and is consuming her in the show. That and after what happened to both Gwen, and Uther, but especially Gwen, it’s pretty much canon substext what happened to Morgana.
Now, it is possible that the exact thing happened to Morgana that year she was gone as it did to Gwen, but personally what I think Morgause did was more subtle. She didn’t intend to make her a mindless puppet, she simply wanted to induce her need for revenge, to feed her bitterness.
Morgana isn’t stupid, she would realize something was wrong, she would have to be locked up. Except. Maybe she didn’t?
Do you remember that bracelet Morgause gave her? The one that is to give her a dreamless night? It’s entirely possible a piece was hidden in it, but more possible, is that it was hidden in her bed. Morgana had all the induced dreams, the ones showing her the hate of those around her, what they would do to her if they found out. What Merlin did. All bad things, all burnings she witnessed. How much Arthur was Uther’s child. And. She never remembered them come morning.
But she remembered a growing bitterness.