Religion has Killed More People than Anything Else?

"Religion has killed more people than anything else", has been tossed around as fact for decades now, but is it true?

Normally, this is pointed at Christians and Muslims, but even if one includes Pagans (Romans feeding Christians to lions and such), all totalled this doesn't even begin to compete with the up to 300 million killed by the Soviet Union (Atheist) or the countless millions killed by science (atomic bombs, poison gases, etc).

Just a thought for today.