This ability to train their owners can include displays of aggressive behavior. Untrained, unsupervised dogs rarely meet their life expectancy. Even well trained dogs will be tempted to chase something interesting, or even disappear into a hole while you are not looking. Off-lead is always a dangerous situation for a Jack Russell unless in a safe environment with experienced JR owners. Many JR's have been killed by cars by darting into the road in pursuit of a squirrel, cat, etc.
Most behavioral problems are due to a lack of companionship, discipline, activity and exercise.
If you've only seen perfect, well-behaved JR's, they are ones that were lucky enough to be exercised, well socialized, and trained. When made to live in a city or suburban-type environment, their needs and instincts do not change. They need a great deal of exercise and outdoor activity, and are usually too loud for such high-density living. Unless your schedule permits many hours at home and a lot of outdoor activity, with a safe place the terrier can run, this is not the dog for you. MANY rescues come from apartments, condos, or homes where the owners work away from the home for long periods of time.