For those of you with dogs as pets; remember, they can kill their owners.
My understanding is that in this case the dog actually belonged to his gf, though the report I read did not say how long they'd been together (and, therefore, how long the dog had known the unfortunate deceased).
It's thought he had an epileptic fit. If so the dog may have been terrified and 'fight or flight' intervened. It's also been said that the dog was well behaved about 6 months ago but his character changed. That leads me to think that the dog may have tried to become the alpha male and this led to the dog taking his opportunity when he saw it.
IMO a pit-bull is the wrong type of dog for anyone except the most skilled/knowledgeable handlers, let alone an epileptic. It's also easy to wonder how the dog was treated when its character changed.