I think paranoid-delusional disorders are even worse. My mother believed for most of her life that everyone was spying on her and out to get her, and she was abusive toward everyone who tried to explain otherwise. She drove away every friend and relative she had (except me - I was probably just too stupid to give up on her). Though there are medications to treat such conditions, just try getting a paranoid to take one. As long as she was able to fool the doctor into thinking she was rational (which she was, about half the time) then nothing could be done.
Eventually, her paranoia did her in - she didn't follow her doctor's instructions to deal with her congestive heart failure, and died last year at 70. It sounds cruel to say it was a relief to everyone in the family...but it was.