Charles Ellicott Commentary 2 Samuel 21:17

"But Abishai the son of Zeruiah succored him, and smote the Philistine, and killed him. Then the men of David sware unto him, saying, Thou shalt go no more out with us to battle, that thou quench not the lamp of Israel." — 2 Samuel 21:17 (ASV)

And smote. —The original leaves it doubtful whether Abishai is the nominative to the verb, or whether it should be simply he, referring to David. 2 Samuel 21:22 seems to imply that at least one of the sons of the giant fell by David’s own hand.

Sware unto him. —This was a solemn transaction, by which David should in the future be restrained from personal exposure in battle. That he should be spoken of as “the light of Israel” implies that his government over all Israel had continued long enough already to make its immense benefits evident.