I have known people who've had Jack Russells. Very few were/are as a nice as Milo seemed to be. The behavior you describe is pretty much what I've witnessed from other Jack Russells. This is an excerpt from a Jack Russell website (and these people like the dogs...)
Is a Jack Russell Terrier the Right Dog for You?
The points made in this Bad Dog Talk have evolved over the years, and represent the worst case scenario of owning a Jack Russell Terrier. Many experienced, as well as inexperienced, dog owners are overwhelmed by the demands of a Jack Russell Terrier, leading to the dogs being abandoned even before they reach adulthood!
Jack Russells ...
... are first and foremost hunting dogs. The traits and skills that make them excellent hunting dogs (i.e., digging, barking, aggressive nature, ability to follow scent) are often interpreted as bad habits that cause people to give them up.
... are bred to go underground, following scent to locate and bark at quarry until they are dug down to or the quarry bolts. If they do not have an outlet for their natural instincts, they will invent new and fun jobs for themselves, which frequently include guardian of the world and/or their possessions and family, chasing cars, hunting birds, bugs or leaves, or endlessly digging in the soil.
... are a big dog in a little body. They have the same need (or more!) for exercise as a much larger dog... and the mentality to match -- they think they are at least 150 pounds, and are fearless, often challenging other dogs three times their size.
... are often aggressive with other dogs. Same-sex aggression and aggression towards other breeds of dogs is well documented with this terrier. It is strongly recommended that no more than two Jack Russells (of opposite sex only) ever be permitted to stay together unattended.
... are NOT a non-shedding breed! All coat types shed! Smooth coats shed the most, dropping hair continuously year round. Rough coats maintain the guard and dead coat, requiring manual shedding a few times a year. The broken coat is an intermediary coat, between the two types.
... require firm, consistent discipline. They are extremely intelligent, continue to test their limits throughout their life. More often than not, train their owners before the owner knows what has happened! This ability to train their owners can include displays of aggressive behavior. Their assertive nature must be understood and handled properly!
... can become very possessive of their owner or a favorite member of the family or of what they consider to be their personal property if allowed to do so to the point of showing aggressive protective behavior that must be controlled from an early age.
... are commonly known to harass, injure or kill other small pets, such as cats, birds, rabbits, mice, rats, etc., simply due to their strong natural hunting instinct. Raising a puppy with a cat does not guarantee the cat's life-long safety!
... remain active well into their 15-year-plus life span; their need for activity and desire to hunt continues for their entire lifetime. Untrained, unsupervised dogs rarely meet their life expectancy.
... require at least basic obedience training. The dog's life may depend on it! 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.
... absolutely need a securely fenced yard! Jack Russells will roam due to their hunting instincts.... even if left in an unsecured for a few minutes! You can let them out every day for three years with no trouble....but one day they will disappear and may never come back. Many JR's have been killed by cars by darting into the road in pursuit of a squirrel, cat, etc. They can also dig under, climb or jump over fences; some can climb trees and any height chain link fencing.
... can be very destructive if left unattended and unemployed! 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.
... are country dogs. When made to live in a city or suburban-type environment, their needs and instincts do not change. It would be unreasonable to expect them to be anything other than what they are genetically bred to be -- a serious hunting dog. Your lifestyle must be adjusted to meet their needs; they must have jobs to perform -- an outlet for their considerable energy and intelligence.
... are NOT recommended as apartment or condo dogs. They need a great deal of exercise and outdoor activity, and are usually too loud for such high-density living. They need room to run; leash walking does not satisfy their boundless energy. 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.
...will NOT TOLERATE even unintended mistreatment from a child. They will not put up with typical child handling such as pulling of ears, tails, etc., or taking or "sharing" of the dog's bones, food, toys, etc. They are very assertive and demanding, and never still... jumping all over whoever will allow it. Jack Russells are not recommended for households with children under the age of six unless you are previously experienced with this feisty little bundle of energy.
... are NOT as they are portrayed in the movies, on TV, or other forms of media. Those dogs are professionally trained and handled, and are very obedient only for VERY short periods of time. Celebrity dogs have their needs met by the trainer, and perform their jobs accordingly.
... require a long-term commitment to obedience, activity, exercise and entertainment... their unique character, intelligence and high energy level can frustrate you, will undoubtedly entertain you, and can bring you great joy (when they're happy!) or great grief (when they're not!). If this type of relationship does not appeal to you, then consider another breed. Jack Russell Terriers are always a work in progress!
Please remember... Russell Rescue is very
busy with displaced terriers because
MANY PEOPLE UNDERESTIMATE LIFE
WITH A JACK RUSSELL TERRIER!
Reprinted with permission from the Jack Russell Terrier Club of America, Inc.