Not the reaction you were expecting

At the start of 2 Samuel, David gets news of Saul's death.
Looking back to 1 Samuel the Philistines were going to war with Saul and as David was allied with the Philistines at that point he was supposed to go with them but the Philistines decided that they couldn't 100% trust David and his men so they sent him back. Thus saving him from the dilemma of fighting Saul himself.
What a lot had changed from when David went out to fight the Philistines- now they are his allies.

Anyway, the one who comes to tell David about Saul's passing is an Amelkite. But he clearly knows that David is destined to be king. This must have been obvious to everyone. He said he saw Saul on the battleground when he had run onto his sword but he hadn't died- he was suffering. So Saul asked this Amelkite to put him out of his misery and kill him. Which he does and then he takes the crown and bracelets and brings them to David.
The Amelkite didn't need to get involved. He could have walked away. He could have kept the trinkets.
I think he thought that David would welcome him as this fellow clearly knew he was destined to be king. Maybe he would get a place in the kingdom.
Instead he was killed as David did not appreciate him killing Saul.