6. The Double Cross
To be fair, this is a tired trope in just about every medium as well; the double cross is so damn predictable in the vast majority of games, and it usually occurs near the tail-end, to surprise the player and give them another enemy to contend with. However, most video games regrettably aren't written with that much subtlety, so it's usually obvious who the turncoat will be as soon as they wander off on their own or clearly have their own, ulterior motive. The other end of the spectrum is that it'll often come out of nowhere and fail to convince, thrown in for mere shock value and little else. Probably one of the better instances is Grand Theft Auto III, which defies video game convention and has the protagonist double-crossed at the very start of the game, with it not being mentioned all that much throughout. Still, it's a crutch of a plot device, an easy one that writers can rely on if they're stuck for character motivation.