Ghazali on Yazeed b. Mu'awiyah
Imam Ghazaliؒ , also known as Hujjatu'l-Islam, was asked whether it was legal to curse the Caliph Yazid b. Mu'awiyahؓ , who is widely considered responsible for the death of the Prophet Muhammad'sﷺ grandson Husaynؓ at the battle of Karbalaa, and his name held in execration by most Muslims. Ghazaliؒ replied that it was absolutely forbidden to curse a Muslim and he who did so was himself accursed. Ghazaliؒ carried his spirit of tolerance to notable lengths when he said;
"How should be allowable to curse a Muslim, when it is not permitted to curse the beasts of the field, and we have been prohibited from doing so?"
"Now, it is certain that Yazid was a Muslim, but it is not certain that he slew Husayn or that he ordered or consented to, his death, and as long as these circumstances remain uncertain, it is not allowable to believe that he acted so. Besides, it is forbidden to think ill of a Muslim, since God has said: O ye who believe! Avoid suspicion as much (as possible): for suspicion in some cases is a sin... [Al-Qur'an, 49:12] The Prophet has declared that the blood, the wealth and the reputation of the Muslim are sacred and of him no ill should be thought.
"Moreover, if any person asserts that Yazid ordered Husayn's death or consented to it, he gives thereby a proof of his extreme folly, for were he to endeavour to discover the true circumstances of the death of such great men, viziers and sultans, as perished in his time he would not succeed not even if the murder were perpetrated in his neighbourhood and his presence. And how can he know the truth (of Yazid's conduct), now that four hundred years have elapsed, and that crime was committed in a place far remote? The true circumstances of it cannot therefore be known and such being the case, it is incumbent on us to think well of every Muslim who can possibly deserve it."
Suppose that there be positive proof of one Muslim having murdered another; the judgment of those whose authority is to be accepted is that the murderer may not be cursed, because the act itself is not an act of infidelity, but of disobedience to God. It may also happen that the murderer repents, before he dies. If an infidel be converted from his infidelity, it is not allowable to curse him how
much the less, then, is it allowable to curse him who repents of having committed murder? Besides, how can it be known that the murderer of Husayn died unrepentant? Know they not that Allah doth accept repentance from His votaries and receives their gifts of charity, and that Allah is verily He, the Oft-Returning, Most Merciful? [Al-Qur'an, 9:104] Wherefore, inasmuch as it is not lawful to curse a Muslim after his death; he who curses him is a reprobate and disobedient to God. [Bukahri] 'Accursed are those who are alienated from God Almighty,' but who those may be is a mystery, except in the case of such persons as die infidels.
As for the invocation of the Divine mercy on Yazid (i.e. رحمۃ اللہ علیہ): it is allowable (جائز), nay (rather) acceptable (desirable, مستحب). In fact, it is included in those words which we titter in every prayer, '0 God! Pardon the men and women who believe', for Yazid.