[He should feel guilty. Not because of Elena, because he's not betraying her. The doppelganger made it quite clear where she stood, and he's doing nothing wrong, in his opinion.
But Stefan knows about Tyler, and while he and the hybrid aren't exactly friends, he's not exactly out to ask Caroline to cheat on her boyfriend.
He might have more of an excuse if Megabot-chan were hanging around, making him deepen the kiss as she does, but it's not and it happens because it feels right, natural, comforting.
The grip on her waist just so as he tilts his head so that he can brush his tongue against hers. No doubt that the kissing bot's requirements are more than fulfilled by now, but he's long stopped thinking about that little detail.]