Never. Dystonias can be painful and disfiguring, but they are never fatal. Some disorders that produce dystonias such as huntington's chorea may be fatal, but they are quite rare. Most dystonias are self-limiting.
No. Dystonia is a description of a disorder of muscle tone. It can affect just a small group of muscles or the entire body. If it affects the muscles of swallowing or breathing, it can result in serious breathing problems. However, with appropriate management, life and function can be quite good. There is no reason to consider the condition as life threatening.