Atheism as a movement has a political agenda as do many who promote it, which want an all powerful state (themselves) to take religion's place (particularly Christianity) in people's lives. During the French Revolution destroying Churches and reliques was a big part of the program, as was redefining history in secular terms (they even changed the name of the months) and to impose their view of liberte, egalite and fraternite, hundreds of thousands of people were guillotinated.
During the Spanish civil war, going to mass or to a religious school were reason enough to be executed by the communist side, and thousands including minors were.
Marx said: religion is the opiate of the populace, then what is communism? Communist death toll during XX Century: 100 million people. Religions usually protect and defend life, they don't preach to kill innocents, but political ideologies do.
Whether individual atheists realize this connection or not doesn't change the fact anymore than individual Witnesses realizing or not TTATT.