זוהר שמות — דף קז ע”א

King David said "against You alone have I sinned" because he sinned only against the Holy One and not against human beings. The taking of Bat-Sheva was permitted since Uriyah had given a conditional bill of divorce before going to war. David's true sin was causing Uriyah's death by the sword of the Ammonites, thereby empowering their idol. Uriyah was a righteous man, called "Hitti" only because of his place of origin. The verse "except in the matter of Uriyah the Hittite" testifies to the gravity of this specific act.