2 Samuel 1:16
And David said unto him, Thy blood be upon thy head; for thy mouth hath testified against thee, saying, I have slain the LORD’s anointed. And David said unto him, Thy blood be upon thy head; for thy mouth hath testified against thee, saying, I have slain the LORD’s anointed.
David treated the Amalekite's own confession as the decisive evidence that made him answerable for Saul's death. The man had openly admitted killing the Lord's anointed, so his own mouth testified against him. On that ground David declared that the guilt of the act and its penalty rested upon the man's own head.