Dark side of the Force
The Dark side is the term used for the manipulation of the Force in an evil or sinister way. Dark side users typically view the Force as a servant to their will or weapon, rather than a counterpart or tool. Infamous examples of Dark side users are Darth Vader and Emperor Palpatine.
In the Jedi Knight series, the Dark side is used by individuals known as Dark Jedi. In Jedi Academy the Cult of Ragnos used the Dark side.