Who killed Absalom, despite David's order to deal gently with him?

Answer: Joab

Joab thrust three spears (KJV: darts) into Absalom's heart as he hung in the oak, and ten of Joab's armor-bearers finished him off.

“Now the king had commanded Joab, Abishai, and Ittai, “Treat the young man Absalom gently for my sake.” And all the people heard the king’s orders to each of the commanders regarding Absalom.”— 2 Samuel 18:5 (BSB)