Now it came to pass after the death of Saul, when David was returned from the slaughter of the Amalekites, and David had abode two days in Ziklag. 2 Samuel 1.1That's right. Second Samuel begins with David returning "from the slaughter of the Amalekites." It's hard to see how David could have found any Amalekites to slaughter since Saul killed them all just a little while before (65), but maybe God created some more Amalekites just so David could slaughter them again. He might have. He's the type.
In any case, when David came back from slaughtering Amalekites, a messenger was waiting for him. And who do you think the messenger was? That's right, an Amalekite!
So I stood upon him, and slew him, because I was sure that he could not live after that he was fallen: and I took the crown that was upon his head, and the bracelet that was on his arm, and have brought them hither unto my lord. 1.10When David heard this, he and all of his men "rent their clothes" (Bible folks are always doing that).
Then David took hold on his clothes, and rent them; and likewise all the men that were with him. 1.11David then thanked the Amalekite for his kindness by having one of his "young men fall upon him."
David called one of the young men, and said, Go near, and fall upon him. And he smote him that he died. 1.15 (We know that God approved of this killing because God approved of all of David’s killings, with the single exception of the matter of Uriah. See 1 Kings 15.5.)