That scene was directly inspired(or taken) from Marlon Brando's One-Eyed Jacks (1961), which the characters shooting hand was smashed, and then once he recovered he staired at his hand and thought revenge.
Going by the filmed movie...Thorgrim was crushing and breaking Conan's hand with his ironboots, as he torments Conan with Rexor keeping the father's sword near with Conan trying to reach for it. Only some parts of this scene/storyarc is in the released print via wide-screen mainly. We're also forced to believe that somehow Doom recognized the boy which is why Rexor was using that particular sword there. As according to that scene, Doom must of also been psychic or remembered beforehand(Vader's Sorcerer clairvoyance?) that he killed this man's parents, before even Conan spoke about it. Then again couldve been just a coincidence as it was Rexor's personal sword by that time, but this doesnt explain why its hung in Doom's chamber in the scripts as a collectors item, and also Rexor changed outfits by the time of the procession to the torture..he was in regular relaxed attire.