Adam Clarke on 2 Samuel 2:21

Take thee his armor - It seems Asahel wished to get the armor of Abner as a trophy; this also was greatly coveted by ancient heroes. Abner wished to spare him, for fear of exciting Joab's enmity; but as Asahel was obstinate in the pursuit, and was swifter of foot than Abner, the latter saw that he must either kill or be killed, and therefore he turned his spear and ran it through the body of Asahel. This turning about that he might pierce him is what we translate "the hinder end of his spear." This slaying of Asahel cost Abner his life, as we shall find in the next chapter.