Norman Osborn descended into insanity as the Green Goblin, with the villainous persona festering in his mind as a split personality. The other Goblins also suffered from mental instability. The Goblin formula that grants each Goblin their superhuman abilities is mainly responsible for turning them all crazy. Norman, in particular, had the formula blow up in his face, while also being aware of his poor treatment of Harry. Combined with Mephisto’s slow corruption of his soul and his encounters with Spider-Man, Norman’s insanity was all but sealed.