Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "When two people insult each other, the one who started will be the sinner until the oppressed exceeds the limits (in retorting)."
If two people engaged in insulting each other, then the sin for that is borne by the one who initiated the insults, because he is the one who initially transgressed, and the other is not sinful because he has the right to answer back and ward off the transgression against him. However, if the wronged transgresses against the wrongdoer, by exceeding the permissible limit in answering back, then his sin will be more than the sin of the one who started.