After seeing that nothing had changed and that Enji was still obsessed with training Shoto to become his successor, Toya psychologically broke, taking this action that all he ever was to his father was a failed and discarded attempt at reaching his selfish ambitions. This mental breakdown caused Toya to completely abandon his old self and become Dabi, a murderous and pyromaniacal villain obsessed with ruining his father's reputation and paying him back for all the suffering he went through