This proposal is about Anti-Mabel from the Gravity Falls book "Gravity Falls: Lost Legends".
Who Is She?/What Has She Done?
Anti-Mabel, as her name suggests, is the exact opposite of Mabel Pines, the main heroine of the story. She pretended to be one of Mabel's allies, but ultimately betratyed her when Mabel's uncles arrived. She plans to destroy Mabel's dimension by any means necessary and to trap Mabel in MAB-3L forever, and would stop at nothing to get what she wants.
Anti-Mabel is so evil, that she was kicked out from her own dimension for being the most evil Mabel in the multiverse. The only villain in my opinion who might exceed her in terms of villainy is Bill Cipher.
She is evil just for the sake of being evil. There is nothing that suggest that there was a disaster that drove her to be this way.
Anti-Mabel is villainous by any way, and she shows no remorse in betraying others and hurting their feeling, as she showed no remorse when she betrayed Mabel.
It is highly implied that Anti-Mabel crossed the Moral Event Horizon by murdering (if not mass murdering) people, which is probably one of the reasons why she was kicked out from her own dimension.
She threatened to presumably kill Mabel after the latter knocked off the former's alien adhesive, which the former stealing it from Ford Pines earlier. She also threatened to launch her "uncles" to the vaccum after trapping them before Mabel arrivedl.
Anti-Mabel can be considered pure evil from any perspective. There is absolutely nothing with which to redeem her. She is one of the most evil Gravity Falls villains ever to exist, and only Bill Cipher can exceed her in terms of villainly.