Sweetie Belle sells her soul, and becomes part demon in the process!
All of Ponyville shall soon be under her iron hoof, and tremble before the might of the very wicked and especially evil Evil Belle!
... Or, not.
Sweetie Belle obtains the Alicorn Amulet and tries to be evil for a day. She isn't very good at it.
The Great And Evil Sweetie Belle returns with a vengeance as the least threatening villain in Equestria.
Sweetie Belle takes a trip to Canterlot with her sister. Now she rules everything. The royal guards are thoroughly baffled.
Sweetie Belle sells her soul to a demon in exchange for very evil powers, and proceeds to (fail to) take over Ponyville.