why did six betray mono

Six betrayed Mono because she realized that Mono was destined to become the Thin Man, a terrifying figure who had previously kidnapped and harmed her. At the moment she saw Mono's face, she recognized him as the Thin Man or saw the inevitability of his transformation into that monster. Out of fear and an attempt to break the cycle of becoming the Thin Man, Six let go of Mono, hoping to prevent him from turning into the monster that would hurt her and others. This act was not out of malice but of fear, pain, and psychological trauma from recent events, including Mono's destruction of her music box, which caused her physical and emotional pain. Six's decision was hesitant and conflicted, showing an internal struggle as she tried to save both herself and Mono in a twisted way by letting him fall.