I think that atheists love having the benefits of a God fearing people, but want to deny the responsibilities of acknowledging the Creator's existence. In the response above the only reason given for not acting like barnyard animals is imagination. IOW, creating order for a groups benefit.
Do atheists believe the behavior of Hitler, Stalin, or Mao was acceptable? They believed they were acting in the best interests of their group. Condemnation of mass killing would be inconsistent with a belief that we are just the sum of our biological parts.
If all that makes us different from the rest of the biological world is our imagination why wouldn't might make right?
Not really. Witness the Middle East. Lots of God-fearing people there, and what advantage are they to anyone? As for might making right, it usually is accepted that it does, although people rarely admit it. Christianity gained a foothold in the 5th century through might, via the emporer Constantine (if I remember correctly.) The winners of wars tend to write the books, explaining why it was morally preferable that they win.
Look at the Old Testament: The Hebrews slaughtered the Canaanites, took the land, and lo, their might was evidence that their God was right (apparently.)