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 Voici, tu me chasses aujourd'hui de cette terre; je serai caché loin de ta face, je serai errant et vagabond sur la terre, et quiconque me trouvera me tuera.