Albert Barnes Commentary 2 Samuel 14:17

"Then thy handmaid said, Let, I pray thee, the word of my lord the king be comfortable; for as an angel of God, so is my lord the king to discern good and bad: and Jehovah thy God be with thee." — 2 Samuel 14:17 (ASV)

As an angel of God—rather, as "the" Angel of God, and therefore whatever David decided would be right.