David killed the messenger

The first verse of Second Samuel is this:

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 1.1

That'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 (68), 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!

I am an Amalekite. 1:8

The Amalekite told David that when Saul was mortally wounded, he asked him to put him out of his misery. So he did. Then he removed Saul's bracelet and crown and brought them to David.

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. 1:10

When 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. 1:11

David 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

The Friendly Atheist - How many people did God kill in the Bible?

David kills the messenger