It depends on mechanism, but many cancer patients do deal with a great deal of pain, especially near the end of the disease process. I am sorry for your experience. It is not uncommon.
The truly sad cases are those in which the patient no longer has decision-making capacity and it falls on a family member to make such decisions. When you aren't the one experiencing the pain, it's much harder to weigh that decision rationally. This is why so many providers push patients to establish legal documentation of their end-of-life wishes.