I don't think Justin felt sorry for him. Not in the slightest. In fact I think he got a bit drunk by this new experience of power. When he met Brian he knew very little about the gay world and he was the bottom (not only literally speaking) in their relationship. Although Brian loved him and came to respect him more and more, he was generally in the lead. Now Justin had the chance to be on top. There was this clueless virgin, who looked up to him. Admired him for his experience and self-confidence. An emotion similiar to what Justin felt for Brian back then. IMO Justin used his chance to become Brian for a few hours, reliving his first time from a different perspective.
But he realized that being like Brian also comes with an unpleasant side effect. To have to get rid of a trick even if that means breaking his heart. It's true. He was very harsh with this guy, but I guess he knew he had to. IMO getting rid of this love-sick trick was a cure for Justin. Who knows better how it feels like to be 'yesterday's f ** k'?
