Charles Ellicott Commentary 2 Samuel 1:15

"And David called one of the young men, and said, Go near, and fall upon him. And he smote him, so that he died." — 2 Samuel 1:15 (ASV)

Fall upon him. —All question of David’s authority to pronounce a capital sentence is here quite out of place. The Amalekite had just recognised him as king, and therefore acknowledged his authority. But, besides this, David and his band of 600 outlaws were accustomed to live by the sword, and to defend themselves against Philistines, Amalekites, and other foes as best they could; and here stood before them one, by his own confession, guilty of high treason.