Redemption is a long and complicated process, as a former supervillain Princess of the Night would tell you. For some, it doesn't come without a major shift in priorites in their life. For others, they simply wish not to change.
Diamond Tiara is in the latter camp, being a spoiled brat of a bully, she had everything she wanted and didn't desire such a thing. She knew it was wrong, but didn't care.
It would take three fillies and a devious scheme to make her see the error of her ways. The Cutie Mark Crusaders would rise and strike fear into her twisted and corrupt heart.