Is kidney failure a painful death if left untreated?

No. Patients in renal failure who are not on dialysis accumulate renal toxins in their bloodstream. This affects brain function, causing the patient to feel sleepy, be confused, and lose their appetite. If a patient has no remaining renal function when they stop dialysis, they will fall into a uremic coma 7-10 days after stopping dialysis, and die within 14-21 days. This process is painless.
Depends. Depends on the cause of the kidney failure and what other associated conditions are present. Progressive kidney failure ultimately will result in a coma, which renders the patient unaware of surroundings and asleep.