This phrase is a common biblical idiom (see Leviticus 20:9; Joshua 2:19; Ezekiel 33:4) indicating that the individual bears full responsibility for their fate. David absolved himself and his men of guilt for the Amalekite’s death, placing the blame squarely on the Amalekite’s own actions and words.
16 Et David lui dit: Que ton sang retombe sur ta tête, car ta bouche a déposé contre toi, puisque tu as dit: J'ai donné la mort à l'oint de l'Eternel!