They were also in the 20th Century, with a much higher population to kill and much more efficient methods of killing and transportation. If the Muslims had this in the 700s, the slaughter of Christians in Europe would have been impressive. But then if the Christians had it, their slaughter of the pagans in that same century would have been equally impressive. Charlemagne would have been much more effective in Northern Europe if he'd had guns, bombs and artillery, and the Muslims would have had a great time.
Yes, but then we're left with the question of why those levels of killing did not go on in lands that did embrace worship of a Creator. If the presence of atheism means nothing but the presence of industrialized weapons does, then why didn't the Christian nations slaughter all their inconvenient citizens? Why didn't we see death camps in those countries, or others where a firm religious mindset was prevalent?
Really? A majority of Stalin's victims were either starved to death or shot, not killed via bombs or artillery.