You know, that would be an awesome plot. Take something that everyone likes, uses, takes for granted. Then, have one strange group of people detest that thing, swearing it's going to take over the world. Everyone scoffs, however, cause they're like THIS is a peaceful little thing. It's fine. We're going to continue using it. Then BAM! they kill people.
Would work really well with cows, IMO, cause when you fight them, everyone will be having a good time going Moo moo moo moo moo!
But could be easily something even more surprising ... like furniture or something.