What is the ruling on criticizing the dead for an act they committed, the effects of which are still present today?

The basic principle is that it is not permissible to curse the dead, based on the Prophet's (peace be upon him) saying: "Do not curse the dead, for they have gone on to what they have presented."

However, it is permissible to criticize the deceased and mention them negatively if there is an overriding benefit, such as if they were an innovator (mubtadi‘) or an oppressor, in order to warn against their innovation or oppression. This is a specific exception to the general prohibition.

Evidence for this is what occurred when the Companions praised and dispraised a funeral, and the Prophet (peace be upon him) affirmed them by saying: "It has become obligatory. You are Allah's witnesses on earth."

Scholars have mentioned examples of those whom it was permissible to mention negatively, such as Al-Hajjaj and Al-Ma'mun, in order to warn against oppression and innovation.

Al-Shawkani mentioned that the hadith remains general in its application unless specified by evidence, such as praising the deceased negatively, criticizing (jarh) discredited narrators, and mentioning the evils of disbelievers and wicked people to warn against them.

Ibn Battal affirmed that cursing the dead is similar to backbiting (ghībah), so if the deceased was an openly wicked person, there is no backbiting for him.

However, if there is no benefit in mentioning the deceased negatively, then it is better to refrain from it.