Most people view growling, snapping and/or biting in a dog to be a sign of totally inappropriate behavior. In fact it's not, and here's why.
Every species has aggressive tendencies. They are part of the way in which we, all of us, ensure our survival. If a man comes up to you in the subway and slaps your face, you're unlikely to respond by kissing him! For our species, that sort of behavior is a clear indication that its perpetrator is dangerous and to be feared. For dogs, many behaviors that we (as humans) demonstrate in our daily lives can trigger the same fight/flight response as that man on the subway. However, dogs have (by virtue of their breed's form and function) acquired a great tolerance for human behaviors. This tolerance allows them to continue to be companion animals to humans. But occasionally, an individual dog (whether because of illness, undiagnosed physiological problem, learning from past experience, etc.) fails to properly perceive the behavior of a human. At that time, one may see aggression from the dog in the form of growling and/or snapping. This does NOT mean that the dog is an unsuitable companion! What it DOES mean is that something is occurring in the dog's environment that is being improperly understood by the dog. The dog is demonstrating a normal reaction to intended (perceived as such) threat.
As a specialist in the diagnosis and rehabilitation of active aggression in the dog, I have seen many dogs whose owners have been advised by trainers or Veterinarians to euthanise. In most of these cases, this advice was totally wrong and unnecessary. Dogs do, for the most part (barring illness, etc.), have very good reasons for showing aggression. These reasons normally grow from the dog's environment and inability to appropriately judge what is really happening. Fully 90% (or more) of the aggression problems I have counseled in the domestic dog is the fault of the environment (and owner), not the dog.
Before surrendering your companion dog to death because of a mistake in judgment, please consult a certified applied animal behaviorist! Dog trainers are not always knowledgeable regarding the subtle cause and effect of complex dog behavior and neither are Veterinarians. One can't know everything! The "behaviorist" with credentials has focused only on that aspect of dog to human interaction. Give your dog a break and a second chance.