Sure watching a rotting corpse walk around is disturbing subject matter. But what's even more disturbing is thinking there is an actual person still trapped in that decaying shell. It's a lot easier to regard a mindless and decomposing eating machine as something that needs to be put down, but when than walking corpse is a loved one who still has their thoughts and emotions from before they died?
Imagine the horror it would be to watch a loved one suffer this fate. Now imagine what that loved one is going through as they watch their own body decay, knowing that death has somehow become worse than life. Add on the eventual stigma a person afflicted like this would receive and how the world would likely treat them. If you were a person whose stature is less than that of an animals in the eyes of others, how would you respond? Accept it? Or fight against it?