‘Ā'ishah (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Do not insult the dead, because they have attained the fruits of their deeds."
شرح الحديث :
The Hadīth clearly stresses the prohibition of cursing the dead and speaking ill of their honor; which is considered from the unethical manners. The wisdom behind such prohibition comes at the end of the Hadīth: "Because they have attained the fruits of their deeds." It means they have already seen the consequences of the good or bad deeds they have done before. Therefore, insulting the dead does not harm them; rather, it hurts the living ones.