What is the consequence for someone who vowed to slaughter his son with a knife and insists on fulfilling his vow despite knowing it is impermissible?

Scholars have agreed upon the impermissibility of fulfilling a vow to slaughter a child. Doing so is among the greatest major sins because it is killing a soul without just cause, and it is a forbidden vow that must not be fulfilled, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Him, let him not disobey Him." And Allah Almighty's command to Ibrahim to slaughter his son was specific to him, for trial and test. This vow is void, and nothing is required of you concerning it. Some scholars have held that sacrificing a sheep as expiation for it is obligatory.