People create violent dogs (intentionally or not). The breed just defines how easy it is to make a dog violent, and how much damage it can do when it attacks a person.
Connected story:
I used to take my dog to an obedience class run by a guy who specialised in training German Shepherds (specifically, police dogs). One day, a woman came along with a huge German Shepherd that was completely out of control. It absolutely adored her, but would savage any other human or dog that came near. She was desperate for help. She had a fiancé who wanted to move in with her! The trainer marched up, grabbed the dog by its collar before it could bite and literally threw it to the ground. Then he planted his foot on its neck and kept applying pressure until it surrendered. When he took his foot away, it stayed lying on the ground. Dog dealt with, he started on the owner.
"Your bloody dog thinks he's your bloody alpha male. Unless you want everyone who comes into your house to do what I just did, you'd better teach your dog his place. Don't let him eat before you do. Don't let him enter a room before you do. Keep a choke chain on him whenever he's around people and give him a proper yank if he so much as starts growling. If he gets above himself, put him on the floor. Do all of that, and you might actually keep him. Because right now you've ruined that dog. And if I was a truly honest man I'd say he should be destroyed before he seriously hurts someone."
She was crying when she left.