Matthew Henry Commentary 2 Samuel 1:11-16

"Then David took hold on his clothes, and rent them; and likewise all the men that were with him: and they mourned, and wept, and fasted until even, for Saul, and for Jonathan his son, and for the people of Jehovah, and for the house of Israel; because they were fallen by the sword. And David said unto the young man that told him, Whence art thou? And he answered, I am the son of a sojourner, an Amalekite. And David said unto him, How wast thou not afraid to put forth thy hand to destroy Jehovah`s anointed? And David called one of the young men, and said, Go near, and fall upon him. And he smote him, so that he died. And David said unto him, Thy blood be upon thy head; for thy mouth hath testified against thee, saying, I have slain Jehovah`s anointed." — 2 Samuel 1:11-16 (ASV)

David was sincere in his mourning for Saul; and all with him humbled themselves under the hand of God, which was laid so heavily upon Israel by this defeat. The man who brought the news, David put to death, as a murderer of his king. In this, David did not act unjustly; the Amalekite confessed the crime. If he did as he said, he deserved to die for treason; and his lying to David, if it was indeed a lie, proved to be a lie against himself, as that sin (lying) will sooner or later prove. In this way, David showed himself zealous for public justice, without regard to his own private interest.