Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 267
Prohibition of Abusing the Deceased without a valid Legal reason approved by Shari'ah

1564. `Aishah (May Allah be pleased with her) said: The Messenger of Allah (PBUH) said, "Do not abuse the dead, because they have attained that which they had forwarded (i.e., their deeds, good or bad).''
[Al-Bukhari].

Commentary:  We learn from this Hadith that we should not censure or condemn the deceased because they have reached their end and they are receiving in the Hereafter return for whatever deeds they did in their life. There is no harm, however, in abusing the disbelievers who died in a state of Kufr.