Hearing of Absalom's death, David wept, "O my son Absalom!" What else did he cry?

Answer: If only I had died instead of you!

David went up to the chamber over the gate and wept (KJV: "would God I had died for thee, O Absalom, my son, my son!").

“The king was shaken and went up to the chamber over the gate and wept. And as he walked, he cried out, “O my son Absalom! My son, my son Absalom! If only I had died instead of you, O Absalom, my son, my son!””— 2 Samuel 18:33 (BSB)