Sins committed openly and with brazen defiance leave a lasting mark that cannot easily be erased. When terrible acts are done in public, they stand as a living testimony that demands a response and resists simple atonement. In Jerusalem, the spilling of blood was carried out so publicly that it appeared almost intentional in its design to arouse the anger of God and provoke His vengeance [רד״ק]. Because the severity of this wrongdoing was so immense, God refused to grant forgiveness [רד״ק].
God responds to this behavior with exact justice. Just as the violence was committed out in the open, the resulting judgment mirrors that exposure. God commanded that the spilled blood would not be absorbed into the earth. Rather, it would remain completely exposed for all to see, as though it had been poured onto a dry, bare rock [רש״י, מצודת דוד].
This deliberate exposure draws on the human habit of leaving an object in a highly visible place so it will not be forgotten. By ensuring the blood remains uncovered, it becomes a permanent memorial before God. It guarantees that the violence will never fade from His sight, standing as a constant reminder for Him to avenge the victims [רש״י, מצודת דוד, ביאור שטיינזלץ]. Within the broader prophetic vision of a boiling pot, this exposed blood takes on an active role. Much like bones placed beneath a cooking pot to stoke the flames, the blood left bare on the rock continually feeds and magnifies the fire of divine wrath [מלבי״ם].