What if Sunset Shimmer left Equestria because of her demon self, so it wouldn't escape and destroy her home. Then she tried to distance herself from others by becoming a bully. Realizing that the demon was growing more powerful she saw no other way but to steal Twilights crown in order to purge herself. However, as time gets closer to the fall formal, the demon quickly becomes more powerful and begins to take control of her actions around the point where Snips and Snails trash the gym. This leads to her more malicious actions after Twilight gets elected. Example, threatening to shatter the mirror. When Sunset dropped the hammer, she regains control for a short period of time and when she puts on the crown she has lost all control.

Sounds good. Ever I have think that Sunset never was really evil.
I see to sunset with a antihero that sacrificed for safe equestria, but she was called traitor. With Itachi of naruto.
Really I would like to see more this kind of stories.

I like the idea too, I would actually also like it that it maybe was one of Celestias first mistakes in training her own student, that she was the one that drove Sunset away by handling her wrong. It is just "one" way to look at it, like that Trixie didn't has to be really at fault when they all blamed her the first time she appeared.