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 And David said to him, “Your blood be on your head, for your own mouth has testified against you, saying, ‘I have killed the LORD’s anointed.’”