Ahh... the Joseph Stalin fallacy.
The argument being that atheism is bad because Stalin was an atheist who was a ruthless murderer. This would be like saying eating bread is bad because most people in prison eat bread and therefore they break the law and end up in prison.
If there were any credence to the Stalin fallacy, there would be a higher trend of atheists becoming ruthless murderers. Instead, what we find is quite the opposite. There is a direct and positive correlation between godlessness and immorality. Or, if you prefer, a negative correlation between religiosity and immorality.
I'm a bit pressed for time at the moment, but I'll be happy to detail this correlation further and provide data to support it should anyone request it.
But the thing that Stalin has in common with the ruthless murderers and amoral religionists in history who have used religion to "evil" ends is ideology. Religion is an ideology. Stalin's version of communism was an ideology. Indeed, Marx would not have approved of Stalin one bit since Stalin's ideology was the very epitome of the hegemony that Marx argued that the peasant class needed to overcome.