If a patient with type 1 diabetes isn't under medical treatment, the disease could lead to death. That is not because of the high blood sugar but because of an acidaemia caused by ketones. The body can not attain energy of ingested carbonhydrates, so it will access it's fat reserves and this causes the production of ketones, an intermediac catabolic product which damages body cells and kills the patient.
A bad metabolic control can cause a disease of the cardiovascular system after some years or decades. A heart attack is a frequent cause of death which results in the transformations of blood vessels because of high blood sugar.