The answer to that rhetorical question, of course, is no one. But that rhetorical question brutally attacks a straw man.
The question should be rephrased: Do any particular belief systems encourage violence or make it more likely for individuals to engage in violence? And the answer to that question is much less favorable to atheists. For Christians have a moral incentive not to engage in violence which atheists do not have – namely, the favor of a deity who encourages good behavior, or the reward or threat of an afterlife.
Atheists, by contrast, have no such moral mooring. The lack of belief of atheists allows individuals to adopt a worldview which encourages selfishness.
Strictly speaking, no one kills for a lack of belief. It is true that one might misuse his religious belief in order to justify violence, just as it is true that no one kills because he don’t believe in anything. But while a person without belief in God might not kill for a lack of belief, he might consider himself more enabled to kill someone than a religiously inclined person because he doesn't believe any moral code is preventing him from engaging in violence.
History has proven that atheists are no strangers to using violence to achieve their own ends, just as it has proven that religious individuals have unjustly killed in the name of their God. Violence, unfortunately, is inflicted by people of all faiths - including those who lack faith altogether.