the problem with geto's mindset didn't make itself apparent when he appeared to "switch sides". the problem had actually been there from the beginning.
when we're introduced to him in the series, his core belief is that the strong should protect the weak. he operates according to this belief and to us, it seems completely fine and acceptable because that's what a person should think, right? it's not untrue that he is objectively stronger than most, so we come to the conclusion that him protecting the weak must be a good thing.
when he goes through his depression era, it appears to us as though his morals are crumbling and his soul is darkening. geto also thinks this, shown in his conversation with haibara.
alongside the viewers, geto thinks he's deviating from the light to the dark. however, when you look closer, you realize that he’s not. nothing actually changes from when we meet him in episode one to when we see him massacre a village.
the problem with geto's mindset is in its fixed polarity. in the beginning, he believes the strong should protect the weak— demonstrating that to him, there are fixed poles of "person in need of protection" and "person who provides the protecting". this can be understood in a subject/agent dynamic, where the weak are agents of their own will, rendering the strong subjects to their needs. in doing this, the weak objectify the strong , turning them into protectors that lack agency.
when he appears to undergo an ideological transformation, the nature of the poles change but their position in a fixed binary doesn't. the subject/agent dynamic merely inverts, where instead the strong subject the weak to their will, assigning themselves agency and stripping the weak of theirs in the process. the poles invert, but the structure that houses them doesn't change— it was there all along, right from the start.
the problem didn't begin when geto decided all non-sorcerers should die. it began when he assigned two distinct groups of people to fixed roles in an established binary, holding steadfast to his belief even when he didn't have a real reason to. his mindset didn't twist, it only appeared to turn something "good" (the strong should protect the weak) into something "bad" (the strong should be protected from the weak).
the binaric infrastructure that scaffolds his belief system actually remained consistent. if you liken his belief system to driving a bus, the bus will still drive as one even if you change out the driver. changing the driver does not fundamentally alter the nature of the bus.
that's why yaga gives yuji such a hard time when he's trying to enrol at jujutsu tech. he's making sure they don't have another geto situation on their hands, where rigid ideals that may appear to help others end up becoming problematic because of their fixed nature.
geto thinks that he's changed, yet he hasn't. he may have replaced the driver, but the bus continues its journey unabated.
