John Gill 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)

But Abishai the son of Zeruiah succoured him
Observing him in danger, made haste to his relief:

and smote the Philistine, and killed him ;
it seems as if Abishai engaged with the Philistine, and killed him; but inasmuch as it will bear to be interpreted of David, and since the four giants here and hereafter mentioned are said to fall by the hand of David and his servants, (2 Samuel 21:22) , it may be thought that this man fell by his hand; seeing it is clear that all the rest fell by the hands of his servants:

then the men of David swore to him ;
after they had observed the danger he was exposed to, and how narrowly he escaped with his life:

saying, you shall go no more with us to battle ;
they had persuaded him not to go to the battle with Absalom; they had suffered him to go with them now, he being, no doubt, forward and pressing to it; but now they were resolute, and determined he should never go more:

that you do not quench the light of Israel ;
signifying that their glory and prosperity depended on his life, and that, should he be taken away, they should be in affliction and adversity, their honour and their happiness would be at an end.

The Targum is, "you may not extinguish the kingdom of Israel," the light and glory of it.