A Pupil of Mine Until He Turned to Evil/Playing With

Basic Trope: A mentor's student pulls a Face Heel Turn and becomes a formidable villain.
 * Straight: Alice's mentor Bob tells her the story of his first pupil, Alex, who turned against his master when his heart was corrupted by power. And That Man Is, of course, the Big Bad now styling himself Lord Killfrenzy.
 * Exaggerated: Not only was Bob the mentor to Lord Killfrenzy, he was also the mentor to Lord Killfrenzy's Dragon and Mooks.
 * Downplayed: ???
 * Justified: Bob is the last Master of a Dangerous Forbidden Technique - forbidden because it can drive the weak-minded mad with power. For Killfrenzy, It's Personal, and for Bob, a new student needs to be around to keep Alex in check once Bob has died.
 * Inverted: Our hero is the best possible Foil to the villains because he was once the star pupil at the Academy of Evil before experiencing a Heel Realisation. For their part, the villains want him dead because He Knows Too Much.
 * Subverted: Alex isn't really "evil", he just disagrees with Bob's use of his powers.
 * Double Subverted: ...because Alex thinks they should be used to Take Over the World.
 * Parodied: ???
 * Zig Zagged: ???
 * Averted: "Lord Killfrenzy? Never heard of him."
 * Enforced: ???
 * Lampshaded: ???
 * Invoked: ???
 * Exploited: ???
 * Defied: ???
 * Discussed: ???
 * Conversed: ???