I think the discussions on use of religion to justify seemingly immoral action is a valid one. However, I would argue that if religion wasn't the vehicle, something else would have been. In fact, I think as culture becomes increasingly secular, this has been proven to be true.
Worst Atrocities of the 20th Century
1
55,000,000 Second World War (Some overlap w/Stalin. Includes Sino-Japanese War and Holocaust. Doesn't incl. post-war German expulsions) 1937/39-1945
2
40,000,000 China: Mao Zedong's regime. (incl. famine) 1949-76
3
20,000,000 USSR: Stalin's regime (incl. WW2-era atrocities) 1924-53
4
15,000,000 First World War (incl. Armenian massacres) 1914-18
5
8,800,000 Russian Civil War 1918-21
6
4,000,000 China: Warlord & Nationalist Era 1917-37
7
3,000,000 Congo Free State [n.1] (1900)-08
8
2,800,000 Korean War 1950-53
8
2,800,000 2nd Indochina War (incl. Laos & Cambodia) 1960-75
10
2,500,000 Chinese Civil War 1945-49
See the link below for the rest of the top 30. While I haven't researched all of them, I think almost all of them were not religiously motivated.
The 20th Century Was the Bloodiest in Human History