And now are you cursed from the earth, which has opened its mouth to receive your brother's blood from your hand;
All Commentaries on Genesis 4:11 Go To Genesis 4
Ambrose of Milan
Indeed, it was not without reason that the mark was set upon Cain, that no one might kill him. Thus it was indicated that evil is not destroyed or removed from the earth. Cain was afraid that he might be killed, because he did not know how to flee. For evil is augmented and amassed by the practice of evil, and it exists without moderation or limit, fights through guile and deceit and is revealed by its deeds and by the blood of the slain, even as Cain also was revealed.