When Twilight is sucked into the illusion of the dark magic door in the Crystal Palace for a few seconds longer than canon, it leaves a lasting effect on her. The ripple effects of this causes her to seek out dangerous magic and put an end to anything that could be abused by the wrong hooves, not realising she herself is falling to the effects of dark magic.
Day 7 of my Self-Imposed 28 Fics in 28 Days Challenge