Well judging by history of every city or country that has been taken over by people who think that God either doesn't exist or doesn't really care what we do about our lives.... I'd say, the score card so far is that men with power who think that man is IT, and there isn't anything to report to at the end of it all.... end up as tyrants.
Now, you maybe not. It's like alcohol. I drink, I don't get drunk and violent. Other folk can't hold liquor at all...they drink till drunk then start fights.
Same with power, fame, money... lots of people can't control themselves... if there was no ultimate accountability except their own sense of proportion...they'd go hog wild. In fact, those types of people HAVE gone hog wild since the dawn of history. Otherwise explain to me why some wars were fought, why men enslaved nations when they could have left well enough alone?
Why risk death when a life of leisure was available and you really don't need that extra town or province after all?
The reality of evil - of evil men doing insane things when given half a chance tells me that if most people in positions of power became atheists, we'd all be in a world of hurt.