Cain understands that his exile is more than leaving his paternal inheritance, hearth and home. He will have to go without God’s face. The worst part is that God does not contradict that (see the rest of the passage). The Bible often speaks metaphorically about hell: fire, pain, etc. Here the reality of hell is depicted: living without God in the worst solitude (see Genesis 4:11).
14 Behold, you have driven me today away from the ground, and from your face I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me.”