Joshua 20:3

“That the slayer that killeth [any] person unawares, [and] unwittingly, may flee thither, etc.] 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 <043512>Numbers 35:12).”

“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. Joshua 20:4”

“20:3 The relatives of a murder or manslaughter victim had the responsibility of seeking revenge for the death. If the avenger (Hebrew go’el) found the killer outside a city of refuge, he could kill him without punishment.”