Many civilizations that eat carbs as staples have low levels of diabetes e.g. rural Okinawans eat considerable amounts of sweet potatoes, rural Chinese eat rice, etc. It is when Western diets high in animal products start to be adopted that levels of cancer and diabetes increases. Carbs may be harmful when you have diabetes, but it is fat that causes diabetes. Fat accumulates in the pancreas (called myointracellular lipids) causing the pancreas to stop secreting insulin, and fat accumulating in or around other cells blocks sugar from entering cells and also prevents insulin from working to transport nutrients into cells. Carbs only cause diabetes if you eat a considerable amount of carbs that it gets converted into fat. However, protein and fat, if eaten in excess, if make you fat as well.

http://www.webmd.com/diabetes/video/kahn-eating-sugar-cause-diabetes