Did Stalin and the others kill specifically for the sake of Atheism?
is a very big yes.
If yes, he would have eliminated people of all faiths in the USSR, in totality. He was clearly capable of doing so, but he didn’t. He killed enough to consolidate his political position. Again, he was more Communist than he was Atheist.
Now that I’ve said this, what do you have to say about the religious background of murderers in a non-Communist country? Are they all Atheists, and in majority?