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. That Man Is, of course, the Big Bad now styling himself Lord Killfrenzy.
Exaggerated: Bob's first pupil goes rogue after only a day of learning the technique.
Downplayed: Bob's first pupil joins a rival team with Bob's teaching, said team is just The Rival and not necessarily evil.
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.
The reason why Alex is considered evil is because he killed Draco, who used to be Bob's most brilliant pupil. However, as the story progresses, we discover that Draco was a double agent sent to infiltrate Bob's dojo and murder him when he least expected it. Alex found out about his plan, and had to kill him in order to protect his master.
Zig Zagged: Bob tells Alice about his former pupil ... but when she tracks the guy down, he's perfectly friendly, and laments the fact that Bob took his branching out on his own so badly. It's later revealed that he's a Rival TurnedAffably Evil, and really is plotting Bob's destruction! Then again perhaps it's just Bob's selfish refusal to accept Alex's independence that created such hate between them?
Averted: Good and Evil are two entirely different sides, no-one on either of which has ever been friends with the other.
Lampshaded: "Ohhh, you guys have an Obi-Wan/Darth Vader thing goin' on? Well, I for one am much more invested in this conflict now that It's Personal."
Invoked: The Big Bad deliberately turned one of his rival's closest allies as a Xanatos Gambit: whichever of them won, Bob would be either killed, emotionally devastated, or at the very least, one ally short.
Exploited: Alex turned away from Bob of his own volition, but the bad guys picked him out because his connection to Bob could become useful.
Discussed: "And this year's Most Likely To Turn Evil And Become A Nemesis Feared All The More Because You Were Once A Friend goes to... Alex!"
Conversed: "Why did Alex cross over to The Dark Side?" "I'm sure they'll do an episode about that."
Played For Laughs: Bob and Alex's pre-battle banter plays on their old relationship:
Bob: So, we meet again. I see you still can't keep your wrist straight... Alex: Shut up! You're not my teacher any more! Bob: Watch that mouth, or I'll have you writing lines. Alex: Wh- You can't... Aaargh!