Does peritonitis always kill you if it isn't treated?

Yes: Yes. Untreated peritonitis is fatal.

Not always.: Peritonitis refers to inflammation of the inner lining of the abdominal cavity, called the peritoneum. This layer is heavily innervated by nerve fibers and it's inflammation causes severe pain. The severity of peritonitis has more to do with what caused the peritonitis (appendicitis, perforated ulcer, diverticulitis, etc). Many of these problems can lead to life-threatening infections if untreated.