Worst way to die = alone.
A while back I did a thread like this and someone posted the name of a disease that resulted in your back arching backwards so steep that they had to severe the muscles in tendons in the back to bring you temporary relief until it started again, this time until breaking.
Also, I would have to say by rabies. If anyone has seen the video of the little kid dying from rabies that is awful. Especially for the parents, knowing that there is no cure.