C. F. Keil and Franz Delitzsch on 1 Samuel 24:14

1Sa 24:14

And even if he should wish to attack the king, he did not possess the power. This thought introduces 1Sa 24:14 : “After whom is the king of Israel gone out? After whom dost thou pursue? A dead dog, a single flea.” By these similes David meant to describe himself as a perfectly harmless and insignificant man, of whom Saul had no occasion to be afraid, and whom the king of Israel ought to think it beneath his dignity to pursue. A dead dog cannot bite or hurt, and is an object about which a king ought not to trouble himself (cf. 2Sa 9:8 and 2Sa 16:9, where the idea of something contemptible is included). The point of comparison with a flea is the insignificance of such an animal (cf. 1Sa 26:20).