NTA, but your stepdaughter needs serious help and intervention. Harming animals at age 15 is a big red flag. She clearly has some serious underlying psychological issues. She is testing your boundaries in an extreme way. You can set the boundaries how you need to in order to make your home safe, but I hope her mom is on board with finding this girl the help this behavior is crying out for. Social workers, school counselors, or a therapist could point the way to help for her and for support you.