The only pet I had that was vicious was an adopted dog that we temporarily had. We tried very hard to keep the dog because it was a foster dog and figured it just needed to learn to trust us. But after a couple months it would still jump up and attack us with full teeth and claws. Mom had to send the dog back because she didn't think it was safe for us to keep.
Other than that I firmly believe domesticated animals are as nice as their owner's let them be. So vicious pets are a reflection of the previous or current owner. I once had to babysit some kids with a vicious dog. They couldn't even play in their backyard because the dog would chase them around the house and pin them to the ground. He was the father's hunting dog, and was great for protecting him from cougars and coyotes but not really a family pet. I also think there is a huge distinction between vicious pets and protective ones. There are guard dogs that are trained not to let strangers in the house or on a property and I don't consider them vicious, even though they can be.
