i.e. I will be an enemy to him, and execute vengeance upon him immediately; because such persons probably would do this in private, so as the magistrate could not know nor punish it. See this or the like phrase Leviticus 20:3, Leviticus 26:17 Jeremiah 3:12 Ezekiel 14:8.
Related Commentaries of Leviticus 17:10
Leviticus 17:10
10 And whatsoever man there be of the house of Israel, or of the strangers that sojourn among you, that eateth any manner of blood; I will even set my face against that soul that eateth blood, and will cut him off from among his people.