John Gill Commentary 1 Samuel 26:16

"This thing is not good that thou hast done. As Jehovah liveth, ye are worthy to die, because ye have not kept watch over your lord, Jehovah`s anointed. And now see where the king`s spear is, and the cruse of water that was at his head." — 1 Samuel 26:16 (ASV)

This thing [is] not good that you have done
Yea, it was very bad, a great fault, and very blameworthy, if he had neglected to set a watch over the king, whose business it was as a general; the words are expressed in a figure called "meiosis", in which less is said than was intended:

[as] the Lord liveth, you are worthy to die, because you have not kept
your master, the Lord's anointed ;
if a watch was set, and these had fallen asleep, and neglected their duty, or had deserted their post; which to do was a capital crime, and deserving of death; wherefore he does not say this of Abner, but of the watch:

and now see where the king's spear [is], and the cruse of water that
[was] at his bolster ;
which he then held up as proofs and evidences of the truth of what be said, that one had been in the camp and had carried off these, and who could as easily have destroyed the king as to have taken these away; and as he came hither with an intent to destroy him, would have done it, had he not been prevented by David.

All which likewise plainly proved the negligence of Abner, in not setting a watch about his master, or the negligence of the watch that was set.