All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Zakat(obligatory charity) should be spent on its eight categories that are specified in the Book of Allah: "Alms are for the poor and the needy, and those employed to administer the (funds); for those whose hearts have been (recently) reconciled (to Truth); for those in bondage and in debt; in the cause of God; and for the wayfarer: (thus is it) ordained by God, and God is full of knowledge and wisdom."{At-Tawbah/60}.
In principle, Diyah-in accidental killing-is due on the Aaqilah(i.e. killer`s male parental relatives), and since the Zakat money is paid by them, the benefit of that Zakat goes to them, which isn`t permissible in Sharia. In such case, they are allowed to pay from the money of charities, not Zakat.
However, if the killer borrowed money and paid that Diyah, then he is regarded as a debtor in straitened circumstances and is eligible to receive Zakat. And Allah knows best.