Someone has come back, and the speaker wants nothing to do with them. Each time the line "When you were here before" returns, it works like a door being slammed on a memory. The speaker remembers a person who thought only of themselves, brushed aside the speaker's wishes like old newspapers, and crushed their spirit as casually as stepping on cockroaches.
The images turn the house into a record of the damage. Footprints and shadows are still there, and they point to the "ghosts" of dreams that died. The speaker doesn't sound shouting-angry. The tone is tired, bitter, and firm, with simple language that makes the accusations feel blunt and final. The sadness is so heavy that it would take an ocean and "maybe half another" to wash it away. That exaggeration shows how deep the hurt goes. The ending is a refusal: you ruined everything once, so you aren't welcome now. The mood is one of grief hardening into self-protection.