WARNING: The following contains spoilers for the Tower of God anime and Fullmetal Alchemist: Brotherhood.
Betrayal can set up some of the most powerful twists in anime, while also creating villains whom fans love to hate. The betrayal of a main character, as in the case of Rachel and Bam in Tower of God, can change the entire trajectory of a story and incite strong reactions from audiences. However, betrayals like Shou Tucker's in Fullmetal Alchemist: Brotherhood are especially horrifying. Neither Rachel nor Shou are particularly likable, but one's villainy far exceeds that of the other.
The Season 1 finale of Tower of God's anime adaptation of the manhwa shocked viewers. The story follows Bam who enters the Tower, an enigmatic structure that people risk climbing for various reasons. By the end, the kind and compassionate protagonist is betrayed by Rachel -- the very person he entered the dangerous Tower for in the first place.
Despite Bam's loyalty and emotional dependence on her, Rachel fully intended to kill the boy when she shoved him out of the protective bubble during a test granted by the Administrator of the Tower's Second Floor. This betrayal cuts deep because of Bam's initial naivety, and completely shifts his goals as he begins to search for answers about the world around him. Though she does not seem to be entirely evil, Rachel is a villain in the sense that her cruel actions directly oppose Bam and influence the story.
However, Fullmetal Alchemist: Brotherhood's Shou Tucker brings new meaning to the term "betrayal". Brothers Edward and Alphonse Elric approach the alchemist for help in regaining their bodies after a failed transmutation, but things quickly go downhill. Desperate to produce an impressive act of alchemy, Shou creates a chimera by combining his young daughter Nina and beloved family dog Alexander. It's also revealed that he had previously created another chimera using his wife years earlier. In the end, Nina's only recourse is to be put out of her suffering through death. Shou's betrayal of his entire family is truly villainous, as his continuous disregard for those around him is blatant and inhuman.
While Rachel's actions in Tower of God are despicable, they make sense from her perspective. Her one and only goal is to climb the Tower and see the stars -- to become a star -- but this dream is destroyed as soon as she enters the Tower. In no uncertain terms, an Administrator named Headon informs Rachel that she is not special and the Tower did not call for her.
Bam, on the other hand, is special and shows his potential for greatness throughout the story, gaining both friends and confidence as he moves forward. He is the hero Rachel wants to be and the star she wishes she could become. Bam may be a lovable protagonist, but he is a formidable obstacle for Rachel. Even though she is ridiculously selfish and wildly misguided by powerful jealousy, there are villains with worse intentions and who are purely evil.
Unlike Rachel, Shou Tucker in Fullmetal Alchemist: Brotherhood is essentially irredeemable. Bam trusted Rachel and entered the Tower to follow her, but he has the ability to survive and thrive without her. Shou's daughter could only rely on her father, the person who was supposed to protect and cherish her. Instead he sacrificed her in the name of alchemical progress, viewing her as little more than a tool. Moreover, before conducting such an unethical experiment on Nina, Shou took the child's mother from her in a similar incident. This demonstrated an inability to reflect on his actions and see how they were wrong, whereas Rachel at least showed hesitation and second-guessed herself up until the fateful betrayal.
Both Shou and Rachel betray those who love them in their bids for self-preservation. Unfortunately, Shou takes this a step further by hurting his own innocent child and repeating his treacherous acts. It's difficult to pity either of these villains, but Shou's cruelty toward his family trumps Rachel's selfishness.