It's not a well-defined question. As pointed out already, this can mean:
1) Can atheists transgress religious law? Yes, since that law exists (at least for some people) regardless of one's adherence to it, or belief in it. They may not be troubled by it, though, or even aware of it.
2) Can atheists have ethics sufficiently solid and well-formed that transgressions can reasonably be called 'sin?' Yes, many atheists do exhibit strong ethical beliefs--often not all that different from the ethics of believers in various systems of religious belief.
Additionally, it's not that far a stretch to interpret the question as:
3) Can an atheist construct a system of belief which has no ethical imperatives that reasonably map onto the concept of sin? Yes, that's possible. But it's also possible to do the same with views incorporating a God--for example, the 'clockmaker' God who creates a Universe but does not interact with it there after would presumably leave a pretty free field for ethical (or unethical!) invention.
And in neither case would that mean that the person in question would necessarily be any more dangerous to know than anyone else. Psychopathy is a condition of the whole personality, not merely (or primarily) a matter of belief.