Satoru Gojo killed Suguru Geto because Geto had fallen too far into darkness and committed mass murder, which Gojo could not justify or allow to continue. Geto's anger and hatred towards non-sorcerers led him to kill 112 villagers, including his own parents, after witnessing two cursed-energy children being mistreated. Despite Gojo's efforts to reason with his old friend, Geto was too far gone and unrepentant, even suggesting Gojo should kill him since "there would be a point to that." Gojo, conflicted but determined, ultimately decided to take Geto's life himself to put an end to his destructive path, sparing him from being taken by authorities, feeling that Geto's cause was now unjustifiable and left no room for redemption.

Additionally, Gojo struggled emotionally with this decision because of their past friendship, and killing Geto meant to Gojo avoiding proving the point that killing may sometimes be justified, which went against his own principles earlier in their relationship. Geto's increasingly extremist views on eradicating non-sorcerers clashed fundamentally with Gojo's ideals, leading to an irreparable rift that culminated in his death at Gojo's hands.

Later, it is revealed that Geto's body was used by a cursed spirit named Kenjaku, which confused many but was distinct from the original Geto who had died by Gojo's hand.