Rival Turned Evil

Two characters are (or were) clearly friends; the innocent lead's friend probably had a dark past or was shunned by others, but nonetheless they managed to become friends.

However at some point, their playful rivalry starts to escalate (a one-sided affair, usually) or they simply drift apart because of the plot. Often times the powder keg is a single event that breaks the increasingly tenuous friendship, at which point one friend becomes the villain.

This can be part of a newly revealed dark past, or dealing with a major problem in a way his friend or other people can't tolerate. Sometimes it's just a very minor misunderstanding no one seems interested in clarifying.

This is a common fate for The Rival and certain types of Lancers, especially if they were the lead character's friend and Missed the Call, or if they grew up with a third, female friend.

If the main character has a strong connection with his True Companions, expect him to make it his mission to turn his friend back to the side of good even while everyone else is trying to kill him.

Whether he succeeds or fails in this mission becomes a source of much drama for the character. Meanwhile the rival turned villain's job is to dredge up all his pent-up feelings of jealousy to try to justify his actions often becoming his own worst enemy in the process.

Examples

 * Eddie Brock is portrayed this way in Spider-Man 3, as fighting for Peter's position at Jameson's office. The Rival/Evil Turn comes after he tries to cheat his way ahead and Peter exposes him; Peter, wearing the Venom suit, goes to a local chapel seeking repentance, while Eddie goes to the same seeking revenge. The church bells ring, disturbing Venom sufficiently that Peter is able to fight him off and Venom drops down a few stories to take over Eddie, who is a much more willing host.
 * Iron Man had his fair share of evil rivals. The first was Obidiah Stane who, in the Marvel Cinematic Universe was an in-company rival to Stark Enterprises as opposed to the head of his own company. He eventually turned evil and tried to kill Stark. In the second film, Justin Hammer was a business rival who ended up releasing an international criminal who had previously killed many people and tried to kill Stark, in order to defeat Stark Enterprises.
 * Sora and Riku in Kingdom Hearts (pictured above). Riku turns to the dark side by Mind Control and is eventually possessed by the Big Bad. After he's freed from this possession at the end of the game, he spends Chain of Memories and Kingdom Hearts II dealing with the consequences. It's especially interesting that Riku fits all three of the marker tropes at the top of the page: he was supposed to have the Keyblade, has very nearly white hair, and grew up with Sora and Kairi.