They're all pathetically easy to manipulate.
Even if their ridiculous shadows hadn't spewed their darkest feelings in a twisted appeal for self-acceptance, he could have mastered them with his eyes closed. Yosuke with his quest for adventure and belonging. Yukiko and her yearning for freedom. Chie, Kanji, Rise, Naoto, Kanji, Teddie, and the multitude of others, all with their own meager problems. And all so transparent they could have had flashing neon signs pointing him to their insecurities. Maybe they did. It couldn't have been easier to play them. His life is a game, and they each embody a stereotype. Sickeningly predictable.
The only thing left for him to ponder is the extent of their trust in him. After he parroted all they wanted to hear, after he took the broken fragments of their lives and stitched them back together, after he tied them around his fingers like string, at what command would they refuse him? At what heinous atrocity would they balk? Or would they follow him to hell and back, pawns at the command of their master?
He hopes there's resistance left somewhere in the deepest pockets of their souls, some refusal over deeds foul beyond their loyalty. It means he hasn't broken them, not yet, for there's no charm in broken toys. But he doubts it. After all, he has already made murder palatable to them. It would only take a crack more to completely and utterly alter them. Was there any victory more complete?
He won't though, not yet, not until he grows tired of the game and masterminds its demise. Until then, he'll keep them, servants for his amusement. After all, what is a king without peasants?