How many millions of people did Hitler, Stalin, Lenin, Mao, Pol Pot kill? They were all atheists. Communist parties like the CCP require all members to be atheists.
And to deprive someone therefore of a meagre 90 or 100 years, the only glimpse they'll have out of potentially hundreds of billions of years underscores the preciousness of life and why it should not be taken.
I will give you that there are surely some who can say fear of gods kept them from killing but in fairness surely there are some who were set free from those gods and realized they didn't have to persecute others just because a religious doctrine says, even if their sympathies said otherwise to the doctrine.
This is false. They most certainly were under the "decree of atheism."
e.g. https://en.wikipedia.org/wiki/USSR_anti-religious_campaign_(...
By this definition, then, you have no beef with most religions
Additionally, it sure looks like atheism as the state religion and exterminating the unbelievers was decreed by the Soviet authorities.
In the case you pointed out, the only thing those atheists had in common with other atheists then and now, in general, is just a name. There is no shared doctrine(s), it is defined by a lack of. I'd argue the converse, that it is much harder to be of toxic nature when your whole belief system, or lack thereof, is defined by a lacking as opposed to the what can be the insistence of the ones who posit beliefs.
If that claim was true, than these monsters who caused the most harm would have been part of organized religion.