this is when religious people like to bring up stalin and mao, or even claim that hitler was an atheist. unfortunately, it's a bit irrelevent because they were killing for their own ends and power, not because the people they were killing were religious. Also, they weren't using atheism as a tool to get others to do their killing.
Well, seeing as Marxist philosophy often includes tenants derived from the philosophy of Karl Marx including such things as, oh...
"RELIGION IS THE OPIATE OF THE PEOPLE"
...and thus makes for an unproductive state, etc., yeah, atheism does in fact factor into a reason for killing people.
It's also key to remember that the maniacal leader does have subordinates, who have their own subordinates, etc., etc... These people often share similarly extreme and often further abstracted beliefs based on those of the leader...
...After all (as an abstract argument), Buddha never claimed to be a deity, yet some forms of Buddhism view him as such. The leader is nothing without the subordinate - devoted followers make the world go round (and fill the mass graves as well).
I want a better argment than the one you've given.