I am a pharmacy student. Eating a "whole slice of cake" most likely won't kill someone with diabetes. Doing so will probably raise that person's blood sugar, and could raise it to a dangerous level if they eat a lot of cake. Having high sugars in the long run can lead to serious health concerns. Consistently high sugars can lead to blindness, neuropathy (nerve pain), and even amputation. Low blood sugars can also lead to complications like fainting, confusion, and in extreme cases death. A healthy blood sugar is 80-130 fasting or without eating for a while, and 130-180 after a meal if you are an adult. Be sure to talk to your doctor about your goals, as they can change from person to person.