One person who wrote a long review of this movie in IMDB said she felt a Great Dane wouldn't have attacked and killed Tony when he was threatening Edith. I disagree. Even though a Dane isn't an attack dog, I suspect they'd be capable of killing to defend their human. I suspect even a Golden Retriever would do the same. Any professionals out there care to comment...dog trainers or veterinarians?
I'm not a vet or a dog handler. However I have had several Great Danes. I can state with experience that they are very protective of their owners. They are very territorial with other dogs, but I've never had one of mine attack a human. It's not impossible though...any abused dog can become vicious to humans. Generally though Danes are aloof but friendly.
I'm no expert either. But having lived with quite a few pure and mixed breed dogs over my lifetime, I would agree that most any dog sufficiently bonded with someone would be likely to do something if "their" person were attacked. However I believe that 99.9% of dogs in this situation are content to stop their efforts as soon as the attack on their person stops. The vast majority of dogs do not keep up the defense to the point where the person is killed. Part of dogs' domestication includes bite inhibition toward humans; when choosing dogs for police/military training, the majority of dogs are declined because they will not bite. So while a Great Dane is indeed protective and certainly has the ability to kill in defense of their person, I think it's a stretch to assume it as a common or probable occurrence.
I loved this movie but don't know how the night club folks did not hear the shot when Edie killed Margaret..