Agreed that eradication of religion and denouncing culture were properties of the soviet revolution, but I disagree that they were primary drivers of the slaughter.
After all, don't most revolutions involve a denouncement of the previous culture? For example, most Americans were British in culture, but they then shrugged off the trappings of that culture, choosing to rule themselves rather than to accept norms set by the rightful king and parliament.
Denouncement of religion isn't new either -- for how many countless centuries have religions (and sects of religions) been denouncing each other? Regarding tolerance, anti-religious secularists are the same as fundamentalists, except that they deny one additional religion. Probably even better morally, because they rarely call for killing those who disagree.