Board Thread:General Discussion/@comment-25266931-20141008232543/@comment-10380701-20141017071546

It's the purpose that they follow. You don't ask a fish why it swims, because that's what it does. It comes as natural to them that they don't need an inherent reason to do it. They just do it because they've been doing it since those scumbag humans were born on Remnant.

There MAY be a reason, like the Mother Grimm being spiteful of humanity because of a deep-rooted history with them, and now her children are eradicating mankind because of that singular mistake.

However, we don't really know that. So the best assumption for now is that the Grimm exist in the same vein that monsters exists in JRPGs: They just attacks humans because they can.