Atheist regimes actually have killed more people - orders of magnitude more people - then everyone else combined. You could argue this is just because they had access to more advanced ways of killing people, but comparing straight death tolls, atheists are the biggest murderers of history.
Napoleon Bonaparte was an atheist “all religions have been made by men” he said. He killed around 6 million Europeans.
Than Shwe - typical atheist dictator of Myanmar/Burma -
Kim Jong Il - leader of the Democratic People’s Republic of Korea - has killed at least 4 million of his own people (we'll never know how many for sure)
Benito Mussolini - Fascist atheist dictator of World War II - he invaded Ethiopia, using poison gas, bombing Red Cross hospitals and concentration camps to kill civilians and destroy “inferior” cultures...
Mao Zedong - atheist leader who killed 20 to 67 million of his “comrades”.
Pol Pot - Atheist Prime Minister of Cambodia - estimated to have killed around 2 million Cambodians (approximately one third of the population)
Joseph Stalin - estimates of the total number murdered under Stalin’s reign, range from 10 million to 60 million. His government promoted atheism with mass propaganda in school, and held a terror campaign against the religious. He crushed the Russian Orthodox Church, leveling thousands of churches and shooting more than 100,000 priests, monks and nuns...
and then of coarse there are the crazy atheist cult individuals like:
Jeffrey Dahmer (who was an atheist - he said “if a person doesn’t think that there is a God to be accountable to, then what’s the point of trying to modify your behavior to keep it within acceptable ranges?”
Jim Jones - Atheist who in 1978, who led 909 people at the restricted communist “sanctuary” he presided over in Jonestown, Guyana, to commit “revolutionary suicide” at his command....
There are crazy extreme people on either side of the fence of coarse, but atheists win if you want to start counting how many people they have killed in the name of their secularist beliefs.