I say, and may Allah grant success: It is not permissible to use zakat to settle the deceased's debt; due to the lack of ownership; as it is a condition for the zakat to be discharged from the debtor's account that it be owned by one of the eligible recipients, as mentioned in Durar al-Hukam 1: 189, and Al-Niqayah p. 52, and Allah knows best.