I can't imagine that there have been too many instances throughout history when an atheist has killed anyone, theist or otherwise, in the name of religion. So having one less reason to kill should automatically affect the number of atheists killed by atheists.
Do atheists kill atheists in the name of God? Oxymoronic way of saying can't happen.
Do theists kill atheists in the name of God? Sure they do.
Plenty of athiests have killed in the name of rejecting God. After all, as atheists like to say, even theists are atheists to some God!