Benny turns heel and goes after Arnold Schwarzenegger's character in Total Recall. We don't want to second guess the writing, but calling your Benedict Arnold character "Bennie" isn't a flash of brilliance.
Dr. Michaels betrays the crew in Fantastic Voyage, attempting to crash their miniaturized ship inside the human body it is exploring. He's pretty much the living embodiment of a hemorrhoid.
Cypher strikes a deal with the Agents from The Matrix to backhand the humans. In exchange, he was to receive ignorance. Which makes us wonder if the blue pill can be slipped into Neo's drink, like a reality roofie.
Another sitcom regular betrays the cause in Aliens, where Paul Reiser's character deceives Ripley in order to preserve alien samples. It's rare that a character is portrayed as evil because they refuse to drop a nuke.
Pettigrew betrayed Harry Potter's parents and joined up with Lord Voldemort. Hey, there's only so many positions for wizards in the fight against evil in this recession, and Voldemort was hiring.
The Lord of the Rings wizard leader turns on Gandalf to grab power using an army of orcs. We, personally, would've gone with an army of Ewoks, because Ewoks and hobbits would be a pretty evenly matched battle.
Secretly collaborating with the aliens in They Live, Holly tries to shoot "Rowdy" Roddy Piper's character. This was a solid film, but it could have been Oscar material had Piper wrestled a space alien.
Hoping to finally stop having to scream his son's name, Michael turns on the survivors in LOST, killing two of them. Because of this, he wasn't allowed to do any fun time-traveling with the rest of the gang.
This high-achieving Vulcan turns on the Enterprise crew in Star Trek VI, forcing Spock to mind-meld with her. You don't want to mind-meld with Spock; everything that guy thinks about is disturbingly confusing.
In the series-ending episode of Angel, Harmony blabs about Angel's secret plans. She also helped Anderson Cooper out Buffy as a vampire slayer, which made us wonder why the U.S. government didn't plan to repeatedly nuke the Hellmouth.