Allah swt says
"That was a nation which has passed on. It will have [the consequence of] what it earned, and you will have what you have earned. And you will not be asked about what they used to do.
(SURAH AL BAQARAH 2 VERSE 134)
Narrated Hz Aisha rz.a:The Prophet (p.b.u.h) said, "Don't abuse the dead, because they have reached the result of what they forwarded."
SAHIH BUKHARI (Vol. 2, Book 23, Hadith 476)
In fact this is command on us that that we should hide the mistakes of dead and show the virtues of them.
Narrated Hz Abdullah ibn Umar rz.a:The Prophet (p.b.u.h) said: Make a mention of the virtues of your dead, and refrain from (mentioning) their evils.
ABU DAWOOD (Book 42, Hadith 4882)
MUSTADRAK AL-HAAKIM (HADITH 1461)
Hz Aisha ra asked in one of the gatherings regarding Yazeed bin Qais May Allah curse him- How is he? (Note that yazeed bin qais openly abused Hz Aisha ra and he was leader of people against Hz Uthman ra) People said he died. Listening this Hz Aisha ra recited Astaghfar. People said you were criticizing him and now you are reciting astaghfar, what does that mean? Hz Aisha ra repled that Prophet peace be upon him said: "Don't abuse the dead, because they have reached the result of what they forwarded.
SAHEEH IBN HIBBAN (HADITH 3098)
Hz A'ishah ra reported the Messenger of Allah (May peace be upon him) as saying :When your companion dies, leave him and do not revile him.
ABU DAWOOD (Book 42, Hadith 4881)
Especially, When by abusing the dead living people feel hurt it is more worse.
Hz Al-Mughirah bin Shu'bah ra narrated that the Messenger of Allah swt said:"Do not harm the dead (and) by that harm the living."(Hasan)
SUNAN TIRMIDHI (Vol. 4, Book 1, Hadith 1982)
If there is no reason then it is forbidden to accuse dead. In fact according to some scholars if by saying something wrong to Non Muslim dead person and if it is hurting living people then even it is not allowed to say something against a Kafir.