The most common pill for diabetes treatment is Metformin, but taking it away wouldn’t cause a sudden death, at least not on its own. It works by increasing the body’s sensitivity to insulin, so it’s used with insulin therapy or with diet/exercise.
Without it, the body’s own insulin and the injected kind aren’t as effective and blood sugar will slowly increase to dangerous levels until it’s high enough to become diabetic ketoacidosis and the patient will fall into a coma. Depending on how sensitive the patient is to sugar, the metformin could be switched with a placebo pill made from sugar which obviously would have the opposite effect.
I remember seeing a while back on a crime show about how a nurse would kill patients as soon as they are getting better by injecting adrenaline into their IVs. Since it’s a natural substance in the body it’s incredibly hard to detect and was for a while written off as natural death until some detective work and heavy EKG scrutiny proved otherwise.