It's fascinating how disciples can evolve into antagonists in stories—I've seen this trope done brilliantly in works like 'The Wheel of Time' and 'The Poppy War'. Often, it starts with a mentor's blind spot; maybe the protagonist overlooks their disciple's growing arrogance or dismisses early signs of moral flexibility. Small injustices—real or perceived—pile up until the disciple snaps.
What really hooks me is when the transformation feels inevitable. Like in 'Fate/Zero', where Kiritsugu's ideals warp Shirou's worldview over years. The disciple doesn't just wake up evil—they're shaped by the protagonist's flaws, the system's failures, and their own unresolved pain. That moment when they quote their mentor's teachings while burning down villages? Chills every time.