Hosseini's A Thousand Splendid Suns seems to suggest that an individual judges right from wrong based on the suffering caused to another person.
In the novel, individuals who are willing to possess the capacity of reflective of thought deem their actions as "right" if it causes suffering to be lessened upon individuals. For example, Mariam's actions of killing Rasheed have to be deemed as "right" because she acted to reduce Laila's suffering. She knew that her actions represented right because the need to help Laila. It is for this reason that she does not even try to escape legal punishment. She judges her actions as right because she based it off of the alleviation of another person's suffering, not in accordance to social notions of the good.
Those whose actions are deemed as wrong under the metric of suffering lack the reflective capacity to judge their own actions. For example, Rasheed is not going to reflect about whether his actions are right or wrong. When he destroys Mariam's teeth by making her chew rocks or when he puts a gun in Laila's mouth, he does not engage in reflection about the nature of his actions. His actions can be seen as wrong because they increase the suffering on his wives.
The actions of the Taliban can also be seen as wrong under the same metric. When they embrace decisions such as public execution by stoning or abusing women that walk alone in public, they increase the amount of suffering that women experience. Those who perpetrate such actions don't engage in the introspection as to the philosophical fiber in their actions. However, the novel concludes that actions that increase the emotional and physical hurt of another person are wrong.