yes suguru's plans to exterminate a vast majority of humanity is undeniably evil, but to say that he is murderous from the very start, cruel for the sake of being so, or lacks compassion or any emotional nuance is a gross disservice to his character's writing.
suguru is a case study of a romantic idealist and self-sacrificial saviour whose absurdly rigid, quixotic ideals are shattered brutally by reality intervening. the intense hatred he has for humanity is born out of, is an inverse of, the intense love he once possessesed for it. this is also why even though satoru is portrayed as brash and selfish and arrogant in the hidden inventory arc, it is suguru that turns "villainous."
suguru places his faith in the goodness of humanity, believes the duty of shamans is to protect the weak, their existence solely hinged upon saving the lives of non-sorcerers, and for that he is disappointed so tremendously, betrayed to an extent that makes it impossible for him to recover his ideals and past self.
ultimately there are also more than one reasons why satoru doesn't become "evil" : 1) "protecting humanity" was never his cause to begin with. he hardly cared about preserving human life, as is evident in his intentions to kill the cultists who cheered on riko's death, and 2) he had someone shielding his inner self : suguru. for it is suguru that tells him the duty of shamans is to protect non-shamans and the weak, suguru who asks him to sympathise with riko, suguru who persuades him to not kill meaninglessly.
satoru is indeed attached to riko, as well. he is the one who decides not to hand riko over to tengen if she wishes to return home, and tries to enliven her last days as a lucid person. it would thus not surpass one’s expectations if satoru turned to villainy post riko's demise, since he never even liked non-shamans to begin with. and yet, he doesn't. suguru protects his heart, which is a part of why he is able to steadily process his grief and anguish over riko's death.
suguru doesn't have anyone to do that for him, he is strong in his own right but not the "strongest", nobody notices how deep of an abyss his soul has sunken in, and he succumbs to the lethal loneliness, falters in this marathon of sorcery.
suguru is brimming with love and compassion: it is what drives his heroism in youth and villainy as a cult leader. he is able to protect gojo's heart but not his own. he fluctuates between two polar extremes : utter distaste of humanity Vs. a duty to protect it despite its horrors. three things serve as final nails to the metaphorical coffin : yuki's words, haibara's death, miminana's abuse. he describes imbibing curses for curse manipulation is "like eating a rag used to clean vomit". how macabre, how grotesque, how enlightening - who is he doing all this for? the humans who killed riko? it was these humans haibara died serving, these same humans violently mistreated miminana.
toji and sonoda encapsulate evil very blatantly, and aren't enough to shake suguru's belief in humanity. but the turning point is the non-shaman cultists rejoicing : suguru is thus forced to confront the banality of evil.
and suguru responds by rejecting what he once loved, embraces the darkness plaguing him. believes the only way to eradicate curses is to uproot their source : humanity. humans, for as long as they will live, will give rise to curses born out of their negative emotions. there is no one to tell him any better, or protect his self-identity. he loses himself to his own sense of empathy, his own ideals.
he isn't indifferent at all, cannot pick and choose whom he loves and doesn't. his love and hatred is collective, in both he gives his all. even amidst his hatred, he doesn't lose his love.
who does he choose to target first, once amassing enough money, power, and reputation? sonoda, the man who ordered riko's assassination. someone who lies in wait to enact vengeance does it out of love. if he was nothing more than a corrupt tyrant, he wouldn't remember the circumstances of riko's demise or care enough about them. suguru's rise as a hero and his subsequent fall as a villain has always been about love. and it seems, to me, up until his death, he prioritizes satoru over himself. doesn't see satoru as a weapon at all, or he would have directly asked satoru to join his cause. instead he poses to satoru a question, presents him with a choice - which in turn makes satoru shaken enough to question his identity, his place in the system, becoming a teacher and dedicating his all to a fitting reformist centrism from an isolated and dare i say, individualistic person such as himself, who stands on the pinnacle of power. but he wouldn't have come to such a conclusion without suguru's experiences shaping his worldview (he himself apologizes to riko during his fight with toji because rather than feeling depressed over her death, he feels the pure pleasure of the world in that moment. killing toji endows him with a sense of duty towards megumi, and riko's death but obviously impacts him, but the change from full apathy, to neutral indifference except in the case of his students, was losing suguru.)
as evil as suguru becomes, he is not a hypocrite. that he kills his own parents is to show the seriousness and conviction he has in his ideals. his code of operation is consistent, even when it turns from pro-human to pro-shaman.
reminds you of what mahito tells yuuji: does yuuji ever consider how many curses he kills? so why should mahito account for how many humans he kills? suguru geto presents us with a possible answer : someone has to care about how many shamans are killed.
you can condemn him for his use of collective punishment, but suguru is a villain!
you can criticize his killing of innocents, but jjk conveys the carefully crafted narrative of a villain who once held staunch traditional and moral ideals.
suguru is evil for proposing collective punishment, but it is incredibly consistent with how emotional he is. he is empathetic because he cares about a girl like riko, doomed by the actions of the rest of the world, forgotten in her misery. he cares and it drives him to the deepest pits of despair, where life loses all color and meaning, despite only knowing her for so long and haibara as well, he enshrines haibara in his memory, when no one other than nanami does. hardly anyone remembers riko's existence, haibara's laughing face, but he does! and for that he spends each moment sinking in the quagmire of his grief and torment. his empathy is a sword of damocles hanging over his neck! to say that he is cruel and unfeeling is to contradict the very agony that drives his (wrongful?) actions. and he is indeed wrong for externalizing this indelible pain, wanting to inflict it upon innocents. but suguru is a villain! has been set up as such!
mahito raises this question to junpei,"is the opposite of love really indifference?" to satoru, it is. but to suguru, it is hatred which is the opposite of love.
