Just some thoughts so far:
-mc is actually the real killer
-wheelchair detective already planned/said to kill him basing from 1st chapter and thought to maybe just have fun with him 1st with detective games
-mc's school female friend hints that mc is quite naturally cunning despite his face/1st impressions. Mc got hurt when she says Mc is kind.
Wheelchair girl will eventually points to mc mother as killer, after that she points to the Mc as killer after they have been proven innocent