It is a very tough question to answer. But I do think these were the reasons:
- She was very hurt because Merlin had tried to poison her.
- She was very hurt by how Uther had behaved and what he had said in The Witch's Quickening.
- She might have been hurt by how Arthur and Guinevere were coming together and Arthur did not pay as much attention to her anymore.
- In conclusion, Morgana was a good woman who had tried to fight for what was right but had been mistreated a lot in the process. This might have been what Morgause used to finally turned her against everyone. So in conclusion, Morgause was most likely the one who played the biggest part in her becoming bad. 09:38, October 2, 2012 (UTC)