Question: Is it regarded backbiting if you talk bad about someone after they have died? Also if s/he is kafir, is it ok to say glad tidings of hell?
Answer: The rules of backbiting apply to the dead, too. The Messenger of Allah (Allah bless him and give him peace) specifically forbade speaking ill of the dead.
Even if the person is a kafir, one cannot be sure that they died like that. Even in the last moment, iman could have entered their heart because of some great good they were doing in their life.
Because of this, we cannot say that specific non-Muslims are in Hell, regardless of what they did.
As Shaykh Adib al-Kallas would emphasize, our responsibility is to worry where we will be in the next life and seek to rectify our actions accordingly; Allah will not consult with us about how to judge people on that Day.
The Beloved of Allah (Allah bless him and give him peace) said, “Make much remembrance of the destroyer of pleasures.” [Tirmidhi, Nasa’i, Ibn Maja, and Ahmad]
Allah Most High reminds us, "O people! Fear your Lord. Truly, the earthquake of the Hour is a tremendous thing. On the day when you behold it, every nursing mother will forget her nursling and every pregnant one will be delivered of her burden, and you will see people as drunken, yet they will not be drunken, but the punishment of Allah will be tremendous indeed." (22:1-2)
Answered by Sidi Faraz Rabbani of the Hanafi fiqh list.