User blog comment:Zathronas/Analysis by an author: World of Remnant, Grimm/@comment-25641997-20141017204013

They don't need to kill humans? Well, that's interesting. Personally, I doubt it's because they're psychotic killing machines and more because they're afraid. They know what humanity can do; they want to fight back before they're wiped out. After all, they do get older and wiser, and the younger Grimm start out quite reckless.

Also, Peter Port must be quite skilled to have captured a Grimm. Hopefully we can see him fight someday.