The literal sense of this reading is that God sentenced Cain to be banned from the ground, to wonder. No where does God take away His presence. It is Cain who declares:
I must avoid your presence and become a restless wanderer on the earth, not God.
In the allegorical sense, Cain’s sin (like Judas’) has a damning effect not because his sin is unforgivable, but because Cain and Judas could not bring themselves contritely to the divine presence, to Jesus they have offended and betrayed.
The moral sense is critical: It’s not just about not killing but it’s also about reparations and humility before God. Cain tries to weasel out of his crime. God issues a judgment and Cain decides its not severe enough, so he’s not allowed in the Divine Presence. This story teaches humility when we have sinned.
Finally, in the judgment, the damned are not the sinners but the arrogant who reject God’s love and mercy.