User blog comment:Morgana High Priestess/Could have Morgause brainwashed Morgana?/@comment-5147915-20130312084104

This is a pretty logical and cool theory. And who knows? It could be true. It would seem a logical explanation for Morgana turning evil had Morgause brainwashed her. However I believe the real reason why Morgana turned evil is that all her trials took their toll against her. For example:

Her finding out that she had magic.

Finding out that she had a halfsister.

Finding out Uther was her father.

Merlin poisoning her to save Camelot and Arthur.

Watching Uther kill those with magic and the druids who had told her she had magic and knowing that the same fate would become her if Uther found out that she had magic.

Mordred almost being executed as a boy.

So there are other reasons as to why she decided to use her powers for evil and not for the good. :)