When he replays that fight in his mind after Starkiller, Kylo realizes that for someone who couldn't use the Force at all like Rey could, Finn wasn't susceptible to being influenced by the dark side of the force. The conditioning didn't work, and Kylo couldn't make him surrender during the fight, or even get into his mind and try to shred him from the inside out. Intrigued, Kylo becomes obsessed with getting his hands on Finn again so that he can dissect him and see what makes him special, if not the force.