Any dog breed can be dangerous if the dog is mistreated or trained that way. If your gf is looking for a dog for protection I suggest a trained German Shepard. I have one and love him to death but there are restrictions with owing a protection dog. Sounds like you and her need to look at what your needs are and speak with a few protection dog breeders/trainers and see if it's really what your looking for.
There are a few types of dogs that have a bit of an aggressive nature. These dogs can be the sweetest companions in the world if they are raised and trained right. Some of them are German Shepherd, Pit Bull, Doberman Pincer and Rottweiler. It takes real dedication to own one of these dogs. You must be willing to put in the time it takes to train them. Do not ever buy an aggressive dog just because you want the status of owning a dog that can rip someone apart.