20th century technology was available to every religion in the 20th century, including the Roman Catholic Church. And again, they are not the ones who murdered millions of innocent people, not even close. Only Atheist Communists have done that.
Were any of those religions in positions of authority, such as the RCC in the Middle Ages? To implement violence, you have to be able to wield authority; RCC authority+ 20th century tech in the Middle Ages, what do you think would have happened? Hmm...
Communist dictators murdered people mostly for pragmatic reasons such as being opposed by them, they didn't do in the name of 'no god' unless you want to make the claim that atheists who were party to it made it loud and clear that they were killing these people because they believed there was no god?