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.

So, just the same we already know, but with changed motives, you say?