Is it permissible to supplicate for mercy for an unjust ruler, such as Al-Hajjaj?

Praying for mercy upon someone who died as a Muslim is permissible, even if they were a transgressor or an oppressor, because their fate rests with Allah. Ibn Kathir reported that Umar ibn Abd al-Aziz did not envy Al-Hajjaj anything as much as he envied him for his dying words: "O Allah, forgive me, for people claim that You will not." As for encouraging prayers for mercy upon him, no; because he is among those who are hated for the sake of Allah, and it is sufficient that we do not curse him by name. Sheikh al-Islam Ibn Taymiyyah said: "When we mention oppressors like Al-Hajjaj ibn Yusuf and his ilk, we say what Allah said in the Qur'an: {Indeed, the curse of Allah is upon the oppressors}, and we do not like to curse anyone by name."