Your answer, "A young Amalekite", is not correct.
The correct answer to
Who killed Saul (in the time of David)?
is
The Philistines
as shown by
2 Samuel 21:12
which is
- KJV
- And David went and took the bones of Saul and the bones of Jonathan his son from the men of Jabeshgilead, which had stolen them from the street of Bethshan, where the Philistines had hanged them, when the Philistines had slain Saul in Gilboa:
- NIV
- he went and took the bones of Saul and his son Jonathan from the citizens of Jabesh Gilead. (They had taken them secretly from the public square at Beth Shan, where the Philistines had hung them after they struck Saul down on Gilboa.)