Jamieson, Fausset and Brown on 2 Samuel 23:6

6. But the sons of Belial shall be all of them as thorns--that is, the wicked enemies and persecutors of this kingdom of righteousness. They resemble those prickly, thorny plants which are twisted together, whose spires point in every direction, and which are so sharp and strong that they cannot be touched or approached without danger; but hard instruments and violent means must be taken to destroy or uproot them. So God will remove or destroy all who are opposed to this kingdom.

2Sa 23:8-39. A Catalogue of His Mighty Men.