Repeating that I'm an atheist, I've always found this a moronic ideal. I've seen plenty of good coming from religious folks, and plenty of evil; ditto from atheists.
If there were no religion, I don't see how the world would just automatically be improved.
I think it is because some militant athiests believe that simply because a lot of people kill in the name of God, that if they stop believing in God they will stop killing.
You perhaps understand this is ludicrous because people always find some excuse to kill each other.