Board Thread:General Discussion/@comment-3257939-20140105070225/@comment-25154376-20140716202346

Or maybe they want to reclaim their land from the humans/faunus. Seeing as how the battle between man and Grimm has been going for ages, no one can be sure who attack whom first.

...That's my in-universe rationale for it. IRL, I just would prefer that the Grimm actually aren't just mindless killers. I hate one-dimensional villains/antagonists in any series; it's just comes off as lazy since that keeps the writers from having to write compelling antagonists.

-Plus it's fun to speculate.