Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.076.

It is not permissible to talk ill of the deceased Muslims or to mention their evil deeds. This is based on Bukhari’s report from ‘Aishah that the Prophet, peace be upon him, said: “Do not speak ill of the dead; they have seen the result of (the deeds) that they forwarded before them.” Abu Daw’ud and Tirmizhi have transmitted, but with a weak chain of narrators, from Ibn ‘Umar a similar hadith that the Prophet, peace be upon him, said, “Mention the good deeds of your dead and cover their evil deeds.” As for those Muslims who openly do evil or indulge in illicit innovation, it is permissible to mention their evil deeds if some public good so requires and as a warning to others in order to discourage anyone who might otherwise follow their bad example. If no such benefit is to be gained, then it is not permissible to mention anything evil about the deceased. Bukhari and Muslim reported that Anas said: “A funeral procession passed by and the people praised the deceased. The Prophet, peace be upon him, exclaimed, ‘It is decided.’ Then another funeral procession passed by and the people said some bad things about the deceased. The Prophet, peace be upon him, remarked, ‘It is decided.’ ‘Umar asked: ‘What is decided?’ The Prophet, peace be upon him, answered, ‘The one whom you praised is entitled to Paradise, and the one whom you described as bad is entitled to the Hell Fire. You are Allah’s witnesses on earth’.”

Cursing the dead disbelievers is permissible, because Allah, the Exalted, says: “Curses by the tongue of David and of Jesus, the son of Mary, were pronounced on those among the Children of Israel who rejected faith.” Qur’an 5.78 Similarly we read in the Qur’an: “Perish the hands of the Father of Flame!” Qur’an 111.1 Pharaoh and others like him have also been cursed in the Qur’an, besides the great curse of Allah about which we read: “Behold! the curse of Allah is on those who do wrong. Qur’an 11.18