I believe the Lord based what he said on this:
10: And the son of an Israelitish woman, whose father was an Egyptian, went out among the children of Israel: and this son of the Israelitish woman and a man of Israel strove together in the camp;
11: And the Israelitish woman's son blasphemed the name of the LORD, and cursed. And they brought him unto Moses: (and his mother's name was Shelomith, the daughter of Dibri, of the tribe of Dan
12: And they put him in ward, that the mind of the LORD might be shewed them.
13: And the LORD spake unto Moses, saying,
14: Bring forth him that hath cursed without the camp; and let all that heard him lay their hands upon his head, and let all the congregation stone him.
15: And thou shalt speak unto the children of Israel, saying, Whosoever curseth his God shall bear his sin.
16: And he that blasphemeth the name of the LORD, he shall surely be put to death, and all the congregation shall certainly stone him: as well the stranger, as he that is born in the land, when he blasphemeth the name of the LORD, shall be put to death. Leviticus 24:10-17
No mercy would be shown if someone cursed the name of YHWH. Not a third person of God but (YHWH) the LORD himself.