You do hear this claim a lot. It’s an understandable one when you comprehend some of their underlying assumptions. Most atheists I’ve known tend to have a remarkably optimistic view of human nature and human potential. (Possibly because the ones that lack that optimism tend to be depressed and even suicidal!)
This is the real source of their problem! If one NEEDS to believe that humanity has a fundamentally good nature and can gradually overcome the mistakes and flaws so readily apparent in our history, then one needs to find and identify an EXTERNAL cause for all those horrific events in history. Since humanity exhibits a stunningly consistent urge towards religious belief, that characteristic makes for a handy scapegoat.
So I think this is the point of discussion to have. If wars and conflicts come about because of religion, then one should expect atheists to have far less conflict, violence, greed, anger, exploitation, etc than one finds in religious people. Is this the case? Good luck finding data to prove it.
The point to make then is that wars come about because of the intractable nature of human fallen-ness, not the external influence of religion. Wars certainly have occurred under the pretense of religion. Most wars have pretenses veneered over the real reasons they are fought. But at root, it is human fallen-ness that causes wars. Our fallen nature is inborn, not indoctrinated.
Don’t expect them to agree with you though. Their worldview and mental health DEPENDS on maintaining their assumption that humanity can rise above our current limitations on our own. If they give up on that belief they can’t have any basis for happiness in life. So expect this view to hold out as long as the atheism does.
But I think it is good to introduce them to the idea of a humanity that is good, but fallen. That while still largely good, we possess a fatal flaw that will destroy us if we don’t get help fixing it. It’s the truth and the truth fits the facts of history better than their scapegoating does.