That's an interesting point. Let me take it from the Atheist point of view. The world operates in such as way that there is numerical set of probabilities that some action or activity will occur which will affect your person. In the case of one person mistreating another person the probability that you will be mistreated as a result has increased. The more you do that the chances increase. This is based on multiple factors not the least being human behavior. It obviously is a safe saying because the more people you meet the eventuality that you will be mistreated will increase therefore "what goes around comes around" is a very good probability.
That doesn't really work. Someone will see one person mistreating another and, as a result of this mistreatment, will say something like "he'll get his one day." That thought isn't the result of the person working out probabilities on his/her head, in fact history proves that there is a high probability that a wicked person will prosper much in this life in terms of wordly things. The thought is illogical according to what an atheist claims to believe.