Numbers 35:26,27

If the slayer—come without the border, &c.— The reason of this law seems to be; because thus the man-slayer was, in some sort, accessary to his own death: for he might have been safe if he had pleased, though, at the same time, Moses in this seems to have indulged the Jews in the hardness of their hearts; for it is what the milder genius of the Gospel will undoubtedly condemn. See Matthew 5:44; Matthew 5:48. And though, in this case, such a slayer was free from the punishment of the law, yet he might be obnoxious to the judgment of God, as having killed an innocent person; see Barbeyrac's Notes on Grotius, de B. & P. lib. 1 cap. 1 sect. 17 n. 4.