The topic statement here is best described by Conservapedia's piece on irreligion/religion and war/peace.  However Conservapedia seems to be having a tough time with cyber/web stuff this week. cached copy
The top line and bottom line here are that irreligion, irreligious ideologies, and irreligious governmental violence such as political Marxism; are very much capable of atrocities and secular mass murder. Atheistic communism and governmental leftist violence caught up in rapid time during the 20th century, killing over 110 million of their own people.