There's something so inherently appealing about seeing someone you love being betrayed in a story. Maybe it's because a good twist like this fools you as much as it does the character, punishing you for trusting even the cuddliest sprites. At that point it becomes more than just you watching a story unfold: it's personal.
It doesn't have to be as big as someone you thought was on your side turning out to be the main villain the whole time or maybe a serial killer with a devilish origami fetish (though that does admittedly help), it could be a betrayal as small as a white lie which transforms into a whole web of deceit that leaves a virtual friendship shattered.
No matter the severity, it's essentially the equivalent of getting Catfished by the game developers - and boy, does it never get old.
10. Lance Vance - GTA Vice City
Rarely does a GTA game go by without one of the thugs that make up your in-game friends betraying you, be it Big Smoke in San Andreas or Dimitri in IV, so you should be expecting to be double crossed while playing Vice City, but it still comes as a huge surprise.
For one, Lance Vance is the coolest dude around, despite being one of the game's biggest liabilities from the beginning. He and protagonist Tommy team up to overthrow Ricardo Diaz, but then the white-suit wearing scoundrel starts to become irritated by his friend constantly berating him and calling out his reckless behaviour. Despite that, you never think a man who invented something as cool as the Lance Vance Dance could ever sink fully to the dark side.
That's exactly what he does though, and Vance eventually sells out his pal to his old crime boss, walking Tommy directly into a trap which ends in a Scarface-eseque shootout. As he says, it doesn't matter if they were once brothers; this is Vice City, and it's only business.