I came across this chart in Moore and Simon (2000-It's Getting Better All the Time). It lists the number of civilians killed by governments, excluding wars:
Soviet Union (total 20th century): 62 million
Communist China: 35 million
Stalin's purges (1930-38): 20-60 million
Germany (total 20th century): 21 million
The Holocaust: 6 million
Cambodia (1975-9): 1-2 million
Based on this track record, governments that suppress religion don't inspire a lot of confidence.