Yes, really

It's unfortunate that some of the constructive criticism is causing you angst, as I'm sure the posts aren't made with any malice intended.

I instruct children, as well, and have two children of my own - one is borderline personality disorder (now, an adult), and the other is ADD (just turned 16).  In my many years of experience, I would hardly consider a child pouting, whining, making "potty" talk, or attempting to take something from another child tantamount to "behaving like an animal."   And, I believe that you could have noticed, seen, or sensed that there were behavioral issues, even if you weren't having the child live with you half of the time.   

If I were in your shoes, I would insist that my husband and his ex rearrange the custody/visitation schedule so that THE CHILD'S FATHER can interact with her and set reasonable limits and boundaries.  I would further insist that THE CHILD'S FATHER and THE CHILD'S MOTHER arrange intensive family counseling for all parties involved.  If you actually despise this kid as much as you seem to infer, the child is definitely aware of it, even if she is only 8 years old.  Just because your child doesn't behave the same way doesn't mean that this little girl is a monster. 

She is in desperate need and it is the responsibility of her mother and father to see that she gets the help necessary to circumvent serious issues, such as:  self-mutilation, eating disorders, antisocial behavior, educational failure, risky sexual activity, narcissism, drug abuse, alcohol abuse, etc.  That you can't define what this child's issues are only further necessitates the prompt action of her father and mother initiating family counseling.  To wait until someone comes up with the Diagnosis of the Day (ADD/ADHD/ETC) is just damning that kid to irreversible damage.  TRUST ME - my son who is BPD was a child just like the "animal" that your step-daughter is, and his father refused to acknowledge that something was wrong and, now that he's an adult, he's socially crippled.