User blog comment:Babysync/Dust/Aura link speculation/@comment-13488655-20130823113704/@comment-13488655-20130823123037

In the real world humans take down many forests and wildlife, it's even calculated that almost every two minutes a species of plant or animal is made extinct in the wild and tons of forests are replaced with buildings and man made structures. It could have been the same in the world of RWBY.

The Grim could have seem humans as a threat because of this and tryed to get rid of them as a act of revenge. (This is just a theory.)