Who came to David claiming he had killed Saul, bringing Saul's crown and armband?

Answer: An Amalekite

David asked why he had not feared to destroy the LORD's anointed, and had the Amalekite executed (KJV: crown and bracelet).

“‘Who are you?’ he asked. So I told him, ‘I am an Amalekite.’”— 2 Samuel 1:8 (BSB)