How bad is it if you don't treat diabetis and you drink beer?

Very bad. Drinking or not beer...Diabetes alone can cause serious sad damage to the kidneys, heart, arteries and retina ( eyes) it may en up with amputations of toes or legs, strokes and heart attracts. Beer will contribute to the big "belly" producing a metabolic syndrome and more resistance to his own Insulin worsening the diabetes.
Bad! Why would you not want to treat your diabetes? It's not going to go away and will only get worst in time. Proper nutrition is important for diabetes and beer are just empty calories with very little nutritional values. If the diabetes is well controlled, you can have beers in moderation so if you want to drink beers, i recommend treating the diabetes aggressively and early.