I wasn’t aware that Jack Russells could be that vicious—I’ve known several, and my dog plays with them at the doggy park quite regularly, with no problem.
Guess it’s all in how they are bred and raised.
Sometimes I think it's more how the “victim” acts than anything else. Some people seem to exude fear of dogs that triggers the dog. Running from a dog triggers attacks as well. I've very rarely had a dog not back down when I commanded them to, even “mean” police dogs. Little yappy dogs though are sometimes just insane.