Geto turned evil primarily due to a combination of disillusionment, personal trauma, and a radical shift in his worldview. Originally a jujutsu sorcerer tasked with protecting non-sorcerers from curses, Geto became increasingly disillusioned as he witnessed the pain and suffering sorcerers endured while protecting unappreciative humans. The critical turning points included the death of his friend Haibara, the realization that curses are created by humans' negative emotions (not sorcerers), and witnessing two young sorcerer girls being imprisoned and mistreated by non-sorcerers. Geto concluded that non-sorcerers were the true source of curses and suffering, and no matter how many curses he exorcised, they would keep being created. This led him to believe that the solution was to eliminate all non-sorcerers to create a world where only sorcerers lived, which he saw as a justified but radical path. This decision marked his turn to evil.