Being turned into a zombie.
OK imagine this, you get bitten, and you have all of the fear and dread of turning and dieing combined, and then you turn, except you don't die. You loose control of your body, you and your body are separated like you are watching a movie, but you can still see and smell and hear and feel pain. and your wounds never heal, and only the destruction of your head ends your life.
So you are forced to watch and experience as your body slowly deteriorates, and as you eat your loved ones and anybody else that gets too close. You are forced to feel all the deep cuts, and stab wounds, and missing limbs, and broken bones as your body does its zombie thing. Only its worse than that, for when you sprain your ankle you walk in a way to minimize main until it heals, your zombie body doesn't do that, so you don't limp from pain, you limp because your ankle can't support your anymore and its flopping around like a wet fish because every step does a little more damage, and since your wounds never heal that is happening to every cut every scratch, and the wounds can only get worse with time. Also you never sleep as a zombie so your sanity is totally gone after a year. After a while when the zombies win, your body can't find food anymore.
So you slowly starve to death exposed to the elements, watching while your body decays and falls apart, and your minds tortures its self. You finally die alone, knowing that you were powerless to prevent yourself from eating everyone you love in the world. Longing for a death that never seems to come.
That is the worst possible way to die.