Charles Ellicott Commentary 2 Samuel 12:9

"Wherefore hast thou despised the word of Jehovah, to do that which is evil in his sight? thou hast smitten Uriah the Hittite with the sword, and hast taken his wife to be thy wife, and hast slain him with the sword of the children of Ammon." — 2 Samuel 12:9 (ASV)

You have slain him. —This is a different and stronger word than “killed” in the first part of the verse, and might well be translated murdered. It was murder in the eyes of the Lord, although accomplished indirectly by the sword of the Ammonites.