What is the ruling on a person making a vow to be fulfilled if an enemy of Islam dies?

Scholars have differed regarding the ruling on vows, oscillating between reprehensibility, permissibility, and prohibition. However, a vow must be fulfilled if it entails obedience to Allah and its contingent condition has been met, based on the Messenger of Allah (peace and blessings be upon him) saying: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Him, let him not disobey Him." Accordingly, if a person makes a vow contingent on the death of an enemy of Islam, and that death occurs, then he must fulfill the vow if it is an act of obedience to Allah.