Is atheism dangerous?

A few years ago I was walking with a friend in a seaside town.  Being Northern Ireland the streets were litter-free, until we turned up a back-alley.  The alley had loads of dog litter along it.  Why did people clean up after their dogs on the boardwalk but not in the alley?  Because in the allay no one was watching.  As an atheist friend of mine admitted, we are all flawed human beings.  When no one is watching, our true nature comes out.  

Now I know my atheist friends deplore the horrors of the twentieth century's totalitarian regimes as much as I do.  I believe they do so because they have a God-given conscience.  And I can't expect them to believe in God simply because he is handy for keeping people accountable.  Yet, I do believe that at the root of the Holocaust, Killing Fields, Russian Pogroms and Chinese Cultural Revolution was a denial of divine accountability.

I was struck by the following words from David Berlinski (contained in John Lennox's 'Gunning for God):

"Somewhere in Eastern Europe, an SS officer watched languidly, his machine gun cradled, as an elderly and bearded Hasidic Jew laboriously dug what he knew to be his grave.  Standing up straight, he addressed his executioner.  'God is watching what you are doing,' he said.  And then he was shot dead.

What Hitler did not believe, and what Stalin did not believe, and what Mao did not believe, and what the SS did not believe, and what the Gestapo did not believe, and what the NKVD did not believe, and what the commissars, functionaries, swaggering executioners,  nazi doctors, Communist Party theoreticians, intellectuals, Brown Shirts,  blackshirts, Gauleiters, and a thousand party hacks did not believe, was that God was watching what they were doing.

And as far as we can tell, very few of those carrying out the horrors of the twentieth century worried overmuch that God was watching what they were doing either."