OT: Problem with my dog

The prey drive is the instinctive behavior of a carnivore to pursue and capture prey. Both German Shepherd Dogs and Boxers have been bred from more prey driven breeds, than say a Pug has. The other aspect is territorial behavior. Wolves will defend their hunting grounds from other wolves. While I doubt your dog is THAT territorial, she probably is to a smaller degree. I would bet that your dog was doing a mixture of those behaviors when she attacked the cat.  Trust me when I say that it was much more traumatic for you than it was for her. It doesn't mean she's going to go on a killing spree, and it's a myth that once dogs taste blood they just want more. Watch her behavior with smaller animals. Is she overly pushy or dominant with them? Does she like to play with toys? Chase smaller animals? You know the way a dog gets when it sees a squirrel running around out the window? That is it's prey drive kicking in. Jus****ch your dog around smaller animals. She may just be a dog that doesn't do well with strange "prey" coming into HER territory. It's all instinctual.