If you have ever been in a position of having to defend your non belief while talking to a believer, you probably have heard the question "Where do you think your morality comes from if there is no god?" Then there is their favorite follow-up question "What stops you from killing another person?" Theists know that the second question is not an easy one to answer but it offers no validity to their way of thinking. It's a hard question to answer because it is a deceptively illformed question. What I mean to say is, murder is not a question of morality, it's a question of motivation.
Think about it. When Police investigate a murder, they look for the person with the strongest motive and not the most immoral person the victim knew. People are motivated by two basic principals; to gain pleasure or avoid pain in one form or another. If the desire for either of the two is strong enough to justify murder, morality (the line between right and wrong) can become severely blurred or even nonexistent.
Theists can say that motivation is guided by a moral compass but is that an arguement they really should be making? When some religious nutjob kills another person for not loving god, not loving the right god or not loving god the right way, they are clearly motivated from both ends. That is to say, hope of gaining acceptence from god and to avoid the pain of eternal damnation. Anyone who can morally justify murdering another person in the name of an unprovable entity is no different that a person with no morals at all.
If believers want to state that atheists can't have morals then they must admit, atleast in certain circumstances, that some believers are twice as motivated to kill than nonbelievers.