Backstabbing the Alpha Bitch

Baroness Rodmilla De Ghent: Jacqueline, darling, I should hate to think you had anything to do with this.

Jacqueline: 'Course not, Mother. I'm only here for the food.

—Ever After

It is a fact of life that, inevitably, the Alpha Bitch has her moment of comeuppance. Just when this moment occurs, the last person you would expect, someone who has always been a loyal member of the Girl Posse, becomes The Starscream and turns on the Alpha Bitch at the perfect opportune moment, inadvertently aiding the heroes in the process.

Sometimes this is to take her place and become the Alpha Bitch in her own right. Sometimes, the posse member suddenly comes to the realization that the Alpha Bitch is simply a miserable human being and not worth the loyalty. Sometimes the character is motivated by some form of Revenge, or pure jealousy, or ambition.

Whatever the motivation, the character's actions will inevitably aid the story's heroine (who is inevitably an outcast and the target of the Alpha Bitch's derision). She ends up doing the heroine's dirty work without the heroine having to get her hands dirty taking down the Alpha Bitch on her own. The character never realizes this, of course, and such a result is the furthest thing from the character’s mind when she moves to betray the Alpha Bitch, but often the character won't object to the heroine benefiting.

Occasionally the character has been pretending to be The Ditz, but is smarter than anyone suspects. Occasionally, the heroine manipulates a loyal member of the Girl Posse into taking this action.

Sister trope to The Dog Bites Back (with which it can overlap if the Alpha Bitch is dumb enough to abuse her own Girl Posse) and Big Damn Villains. Also sometimes overlaps with The Starscream, though it is much more frequent that Backstabbing the Alpha Bitch comes out of nowhere and the Starscream is usually obvious from scene one. (And if the Alpha Bitch in question is really a Lovable Alpha Bitch, that makes this character an Evil Chancellor.) Contrast Professional Butt-Kisser.

Examples of Backstabbing the Alpha Bitch include:

Anime

In Revolutionary Girl Utena, the Alpha Bitch Nanami gets backstabbed by her Girl Posse (Keiko, Aiko and Yuuko) as part of her breakdown. The Starscream girl member ( Keiko Sonoda, the one with Girlish Pigtails) remarks that it felt good to finally tell Nanami how they all felt about her, that they had gotten tired of exploiting her popularity, and then takes over the gang. Nanami's recent treatment of the girl, based in jealousy, didn't exactly help her.
This happens to Manami in Life, when one of her Girl Posse tells the teachers of her bullying. This causes Manami to bully her, to the point where she attempts suicide.

Film

Tia from Saved waits until the prom to destroy Hilary Faye's reputation by proving that she performed the vandalism on the school for which the heroine was blamed.
Heather Duke from Heathers: After Heather Chandler's death, Heather Duke quickly becomes the new queen bee. Of course, Heather Duke morphs into this role herself rather quickly.
Lisa Luder in Romy and Michele's High School Reunion waits until the reunion before confronting queen bee Christy Masters. Lisa seems to be motivated more by moral outrage and personal dislike of Christy than traditional jealousy.
- The flashbacks show Lisa had always been a more independent thinker than the rest of the group.
Cady in Mean Girls is an odd case of the heroine doing her own knifing.
Jaqueline, the "ugly" stepsister in Ever After, rebels against her mother and sister's shameless vampiness after realizing they considered her just barely higher on the family's totem pole than Danielle (who they had made a servant in her own home and denied all happiness) and after being casually dismissed as never being able to attract any royal's attention at the ball ("You're only going for the food"). She wisely decided to help Danielle dish them out a karmic comeuppance.
- She's also the one who patched Danielle up after the whipping and expressed the opinion that her mother and sister had crossed a line, well before there was any profit for her in doing so.
Tiara Gold in High School Musical 3 does it to Sharpay, but it ultimately backfires.
From Amanda Byne's Sydney White, a modern retelling of "Snow White", sorority president Rachel Witchburn, after much verbal and emotional abuse to several people, including her sorority sisters, finally got stripped of her presidency and her regime by said sisters.
In The House Bunny, the Zetas are just one pledge short of the minimum, so the Alpha Bitch demands they be shut down. Just then, her much-abused friend decides she's finally had enough of being belittled and punched in the chest, and announces that she will pledge Zeta as well. (Made all the more satisfying when the Alpha Bitch is herself punched in the chest at the end of the scene!)

Live Action TV

In 8 Simple Rules, one of the sisters explicitly invokes this trope.
In Glee season 2 former head cheerleader and Alpha Bitch Quinn tells Coach Sue Sylvester of New Cheerleading Captain Santana's boob job to get Santana demoted and put herself back on top.
Buffy the Vampire Slayer: Harmony does this to Cordelia, after it becomes public knowledge that she's been dating Xander.
Kamen Rider Fourze has Miu being attacked and sabotaged by one of her Girl's Posse in a beauty pageant. The latter became a zodiart monster, too.

Web Comics

Coming Up Violet, swapping the spiked and non-spiked drinks.

Western Animation

Both of Heather's "teammates" in Total Drama Island eventually abandon her... though her near Contractual Immortality doesn't allow them to sink her or survive much longer after the revelation. Still, she gets points for growing a spine.
Happens to Azula from Avatar: The Last Airbender during "The Boiling Rock", when Mai and Ty Lee turn on her. As Azula thought she held total command over them, their betrayal makes her paranoid of everyone she commands, and is one of the contributions to her Villainous Breakdown.
Anastasia from Cinderella 3 does this to her stepmother and stepsister.
In the Recess episode "Outcast Ashley", Ashley B. and the other two Ashleys kick Ashley A. out of the clique for forgetting "Purple Day" (the anniversary of the day they first met where they all wear purple), and so Ashley A. decides to hang out with Gretchen for a while. However, the other Ashleys end up befriending her, and then in the end, they leave Gretchen behind and Ashley A. returns to the group.