Joshua 20:3 - Matthew Poole's English Annotations on the Holy Bible

Unawares and unwittingly, Heb. through ignorance, or error, or mistake, and without knowledge; the same thing twice repeated, to cut off all the claims and expectations that wilful murderers might have of protection here; and God having declared that such should be taken even from his altar, that they might be killed, Exodus 21:14; and accordingly Joab was by Solomon's order killed even at the altar, 1 Kings 2:28-31,34. It is the more strange and impudent that any Christians should make their sanctuaries give protection to such persons whom God hath so expressly excepted from it, which the papists do; and their doctors are not ashamed to defend it upon frivolous reasons. The avenger of blood; the kinsman, who had right or power to demand or take vengeance of the slaughter.