Tokyo Hotel's Favorite

Can someone give me an explanation about the story of chap 51 ? At the end after the mom died, requested by Guushi’s (or Gushu ion know, i hella forgot) parents , Usobuki said the parents added an additional request to kill the son and he gave him coffee and hypnotized him that it was poisoned and the poison will spread in the body . But how come at the end the guy was still alive and came to kill his adopted parents ? If there is something I didn’t understand at the end of that chapter, someone can explain to me plz !!