Thing is with Kirsty, is that they've made her a rounded character, it's not just 'she beats up Tyrone cos she's evil'. You can see the family you grew up in, and whilst it doesn't forgive her behaviour it does make you see she is a three-dimensional person and not just a monster. Plus, she was paranoid when she first thought Tyrone and Fiz were together, but now she's been proven right and it must be devastating for her when Tyrone has been acting like everything is fine and he wants to marry her.
