Is dehydration a disease that can kill you?

Yes. Patients (mostly children) often die in third world countries from diarrheal diseases, not because of the infection but because of the dehydration caused by the diarrhea and poor access to clean drinking water. Without proper hydration, the organs shut down especially the kidneys and brain. With electrolyte abnormalities, it is possible to have abnormal heart rhythms which can lead to death.