Let's face it, these dogs (and cats) cannot help being who they are; their impressions of the world have been shaped by events not of their choosing. (Can we say the same for human behavior?) Nevertheless when living and closely interacting with humans (and innocent children) daily, any behavior that endangers human health and safety is unacceptable.
My experience during thirty years of working with dogs and cats has taught me that many well intentioned people, certain that their gentle and loving ways will modify the behavior of the fearful/aggressive dog or cat, have learned a hard lesson in animal behavior.
Often the "saviors" of these animals have been injured and even psychologically harmed when they learn that all their love and understanding will not correct the aggressive animal's behavior.
I am not saying that all dogs and cats with fear/aggression are lost causes; I am saying that a great percentage of them will continue to be a danger to human health and safety no matter who or what attempts to modify the behavior.
So, what is an owner to do? Consult with your DVM, breeders, and animal shelter personnel about your particular dog (or cat), maybe even spend a little money on consultation with a professional animal behaviorist about your pet.
If you choose to keep the pet and attempt behavior modification, be prepared for the experience to dominate your entire home life. Every family member will have to contribute to the plan of action and it will be a 24 hour-a-day experience; that dog or cat will be the focal point of your thoughts and activities.
Are you willing to do that? Should you do that? I have witnessed many sincere and vigorous attempts to modify fear/aggression in dogs and cats that have left the animal's caretakers frustrated, demoralized and injured in their failed attempts to pacify the pet.
At the heart of the problem is the fact that the animal can't help being who s/he is! S/He can't reason that the owners do not represent a threat or that the stimulus triggering the fear/aggression is not a real danger ... he simply acts and responds as ordered to by a brain that was imprinted with certain directions that the animal will never be able to modify.
Many, many times I have been a part of counseling owners about this fear/aggression problem. If we can rule out and are certain that the animal does not have anything physically wrong that may be triggering pain or discomfort, such as bladder stones, gastrointestinal foreign bodies, tumors or infections, and we are certain that the behavior is personality based, the choice may be to euthanize the unfortunate pet.
Even if the pet is "OK most of the time" and only a threat two per cent of the time...is that an acceptable risk for the family to take? If the cat only scratches someone's eye occasionally or only bites severely once in a while, is that acceptable? If the dog only attacks "certain" people or gets frightened only by small children necessitating the continuous separation of small children from the dog ... is that an acceptable risk to have living in your home all the time?
Regrettably, I have seen far too many empathetic and sincerely-intentioned pet owners make excuses for their dog or cat's harmful behavior. I have seen children scarred from dog bites that have occurred well after the dog has bitten the child or others in the past. Some pet owners really go too far in excusing the dangerous behavior of their dog or cat, blaming everything but the dog or cat, and these owners fail to see the improper and dangerous priorities they have set.
In the case of a dog or cat being a real threat to human safety, you must set aside emotional attachment and look at the situation objectively. You must ask "No matter how much I love this animal, is it a danger to human health? Am I, as the caretaker and person responsible for this animal, willing to gamble that it won't ever tear out someone's eye, bite off someone's nose, scar someone's face...or even worse?" You be the judge ... and then you live with the consequences of your choices.
I have had entire families come with their pet to my animal hospital where everyone is crying and completely emotionally drained by the absolute necessity of euthanizing their pet simply because the dog or cat has demonstrated itself to be a danger to them and others. Nobody wins in these situations...not the family members, not the pet, not the veterinarian. Simply put, the animal cannot help being who it is. Unfortunately, who it is can be a danger to human health. It's a no win situation for all involved.
And to give away a pet with fear/aggression personality traits to someone else is not a solution. The innate tendencies of the animal evolved from genetic predispositions and early brain/sensory inputs. You can't help that -- and neither can the dog (or cat).