How should I approach this topic with my daughter?

she is 19? She's an adult, you want to teach her that she can't do that to people, then CALL THE POLICE and file charges against her for abuse and battery. It doesnt matter what happened int he past, this is unacceptable behavior and if you dont show some serious tough love she is going to really hurt you or someone else....call the dang police, go to the hospital and be seen and document the abuse......allow the behavior and you become the enabler of the abuse and her children will learn it too