My little headcanons for how ayano became a bully is,that when she got heartbroken by taro the genderbend versions of the bullies saw her and they took her in to their group and after a while they started developing feelings for her.
They know that she was (and still is) a murderer and they help her with the murders but instead of killing for taro she starts killing for them because she also started developing feelings for them ever since they took her in and made her a bully.
Should I draw ayato as a bully?