Both, because robber is a synonym for thief, and thief is a synonym for robber he is not a thief.
Luke Castellan, a son of Hermes.
From the Cary Grant film (1955) and the Robert Wagner TV series (1968), the theme is "it takes a thief to catch a thief".The original proverb is "set a thief to catch a thief".
Luke was... He hid it in the shield he gave Percy....
A thief is a name for someone who steals, a criminal so to speak.