People will still fight over other reasons. Just because there's no longer terrorist attacks on Muslims or Christian attacks on abortion clinics, doesn't mean that people would live in harmony. People would still fight over land, fame, resources and greed. Atheists will fight other atheists over matters i mentioned above.
I am seriously pissed off over atheists who claimed that "the world would be better off without religion". Use some logic and common sense for crying out loud!