Is it permissible to curse the unjust ruler, and is injustice alone a reason for cursing, and what are the other acts that necessitate cursing?

Injustice is forbidden, and its perpetrator is threatened with curses. Among the deeds that are threatened with a curse are dying in disbelief, concealing knowledge, a person claiming lineage other than his father's, committing injustice in Medina or harboring oppressors therein, and leaders failing to rule justly or fulfill their covenants. The texts regarding promises and threats (of punishment) have conditions and impediments; thus, the threat (of punishment) does not materialize unless the conditions are met and the impediments, such as repentance and good deeds, are absent.

It is not permissible to curse a specific Muslim, even if he is a transgressor, for cursing a believer is like killing him, and if a curse finds no deserving recipient, it returns to its utterer. Scholars used to curse those whom Allah and His Messenger cursed generally, but they did not curse specific individuals, because cursing falls under the category of general threats of punishment, which cannot be definitively applied to a specific person due to the possibility of repentance, expiating good deeds, atoning calamities, or accepted intercession.