Without God people are left to decide what is wrong or right. That sounds great to many people because they know what is wrong and right. Oddly, people seldom agree on this. Furthermore, when people gain power over others their view of right and wrong tends to change to right being whatever protects their power or grants them more power. Historical cases of this are:
Ukraine Famine | United Human Rights Council
and
Chronology of Mass Killings during the Chinese Cultural Revolution (1966-1976) - Online Encyclopedia of Mass Violence
Religious death tolls happen, but they tend to be relatively small and with good reason, ie crusades to kick out foreign invaders from Europe or the inquisition to root out subversive elements of society ie Kabbalists who were accused of sacrificing Christian children.
Ukraine Famine | United Human Rights Council
and
Chronology of Mass Killings during the Chinese Cultural Revolution (1966-1976) - Online Encyclopedia of Mass Violence
Religious death tolls happen, but they tend to be relatively small and with good reason, ie crusades to kick out foreign invaders from Europe or the inquisition to root out subversive elements of society ie Kabbalists who were accused of sacrificing Christian children.