Is it permissible to give from zakat funds to a killer sentenced to pay blood money (diyyah) in order for him to pay it?

If the killing was accidental, the blood money is to be paid by the killer's 'Aqilah (male relatives on the father's side). If the 'Aqilah are unable, then it is from the Muslim treasury (Bayt al-Mal). Otherwise, it is incumbent upon the killer himself.

If the killing was intentional, the blood money is incumbent upon the perpetrator himself.

If blood money is decreed upon the killer and he is poor and unable to pay it, it is permissible to give him from the Zakat because he falls under the category of Al-Gharimin (those in debt), according to the Almighty's saying: (Indeed, [prescribed] charitable donations are only to be given to the poor and the needy, and to those who work on [administering] it, and to those whose hearts are to be reconciled, and to [free] captives, and to those in debt, and for the cause of Allah, and to the stranded traveler; a duty ordained by Allah. And Allah is All-Knowing, All-Wise.) [Al-Tawbah: 60].

Sheikh Muhammad ibn Ibrahim and Sheikh Ibn Uthaymeen, may Allah have mercy on them both, issued fatwas to this effect, provided that the one upon whom the blood money is due, or his 'Aqilah, are poor.