Side Switching

Several characters from the show switch sides, intentionally, or not.

The pack

Xander is the first to go evil. Possessed by hyena spirit, he becomes pretty primal, killing the school mascott and attacking a family as well as Buffy and Willow. Giles manages to break the spell and revert Xander back to his normal self.

Surprise/Becoming

After finding love in Buffy's arms, Angel looses his soul and reverts into his former self. he is let loose on a killing spree in Sunnydale while Buffy finds the courage to defeat him.
Angelus kills several of Buffy's school mates and Jenny and plans to awaken an ancient demon. Buffy manages to stop him but sends him to Hell in the process.

Consequences/Graduation Day/This year's Girl/Who Are You

During the third season, Buffy has to deal with a rogue Slayer. Called after Kendra's death, Faith is a rather wild young woman tortured by oh so many issues. Tired of sharing the spotlight with Buffy, her behavior becomes more and more wreakless, eventually killing a man.
The Council messes up and ends up loosing Faith, you teams up with the Mayor? Sent to hospital in a coma by Buffy, Faith wakes up eight months later and sets out for revenge. After playing around a bit with Buffy she ends up in L.A. and fights Angel. He manages to reason her and she decides to turn herself into the police.

The Wish/Hells Bells/Selfless

Anya has switched sides more than once. Formerly a young human toying with magic, she becomes a vengeance demon after turning her unfaithful boyfriend into a troll. For over a thousand years she traveled the world granting the wishes of scorned women.
But when the scorned woman is Cordelia and her wish sends Sunnydale into Bizzaroland, Giles strikes back, breaking the spell and making her human once again.
Three years later, as Xander leaves her at the alter, Anya becomes a vengeance demon once again, although much 'softer' than she was before.

The Initiative/Grave

Once a vicious vampire, Spike is forced to switch sides when a chip in his head prevent shim from hurting humans. He joins up with the Scoobies and helps them kill vampires and demons, saving them more than once.
In love with Buffy, he even gets his soul back. Tormented by the first, he starts killing again, but Buffy helps him out and he continues fighting with her.