| Gill's Exposition of the Entire Bible That the slayer that killeth any person unawares, and unwittingly, may flee thither,.... Who through mere accident, and without design, killed a person, friend or foe, one of his own kindred, or a stranger, without any malice against him, or intention to take away his life: and they shall be your refuge from the avenger of blood; from any of the relations of the deceased, who might be stirred up to avenge the blood of his kinsman on the slayer; see Numbers 35:12.
Geneva Study BibleThat the slayer that killeth any person {a} unawares and unwittingly may flee thither: and they shall be your refuge from the avenger of blood. (a) At unawares and bearing him no grudge.
Wesley's Notes 20:3 Unwittingly - Heb. Through ignorance, or error, or mistake, and without knowledge. The same thing twice repeated to cut off all the expectations that wilful murderers might have of protection here; God having declared, that such should be taken even from his altar, that they might be killed. It is strange that any Christians should make their sanctuaries give protection to such persons whom God hath so expressly excepted from it! Avenger - The nearest kinsman, who had right or power to demand, or take vengeance of the slaughter.
Scofield Reference NotesMargin avenger Heb. "goel," Redemp. (Kinsman type). See Scofield Note: "Isa 59:20".
Matthew Henry's Concise Commentary20:1-6 When the Israelites were settled in their promised inheritance, they were reminded to set apart the cities of refuge, whose use and typical meaning have been explained, Nu 35; De 19. God's spiritual Israel have, and shall have in Christ and heaven, not only rest to repose in, but refuge to secure themselves in. These cities were designed to typify the relief which the gospel provides for penitent sinners, and their protection from the curse of the law and the wrath of God, in our Lord Jesus, to whom believers flee for refuge, Heb 6:18. | 
