"I'll be Great and Powerful! For I am Trixie. I shall be the best, and nopony will stop me. Did you hear me? Nopony!"

She is prideful, She is reckless, but she does not mean to be so. She just wants to feel comfortable with herself, Doesn't she?
Slaughter And Salvation, are two faces of the same coin. Even through their motivations are never revealed, the emissaries they choose are tied to feelings. When you are the opposite of a hero, what would you expect to be chosen for?

Maybe, your dreams can guide you to something very Great and Powerful.
[Alternative Universe: Happens after Discord's reformation, without any other canon reference after it]