Zombies don't deal in gray areas; when it comes to gray matter, there is a question of appetite (the idea of zombies eating brains was popularized with the satire, Return of the Living Dead), but ambiguity just isn't their specialty.
Maybe this is why we love them, because of an innate desire for life with no moral ambiguity. When your existence has the absolute value of consumption, things tend to get pretty simple.
The elemental nature of zombie attacks is simple, too: survivors fight back or get converted to just another face in the crowd, a death sentence in a world where death has no meaning.
Serious people are talking about our chances of repelling a zombie apocalypse, and they aren't good. This begs the question: if we desire moral absolutes, why not surrender to the horde and achieve the impossible dream?