Curate, connect, and discover
I see a lot of people talking/writing/musing about how Riko's death was a starting point of Suguru's downfall.
But I, for one, think that foundation for Suguru's deflection had been laid way prior his meeting with Riko. It was the way he had been taught (or simply grew up) thinking about sorcerers and non-sorcerers.
From the very beginning he had labeled non-sorcerers as "the weak", those, who need to be protected. The weakness in question being the inability to use cursed techniques and manipulate curse energy.
Ultimately, despite his initially noble and relatively collected facade, he'd been showing signs of being a classist (or racist? Considering how later he came up with the thought that sorcerers and non-sorcerers are two different species entirely) from the very beginning. He wasn't "a sincerely kind and good person", he chose to act like one, maybe because he was told to, maybe because he was youthful (and who doesn't want to be a hero, when they're young?) and untainted, but not flawless still, and thus instead of being straight up cruel and genocidal, he used to be patronizing and lowkey condescending alongside with being nice. Non-sorcerers always were a second class people for him. It's just before Riko's death he'd been wearing the mantle of the "noble protector", and after her demise and finding out how Nanako & Mimiko had been abused by the hand and will of those who, he'd thought, needed his protection, he snapped and spiralled down to straight up hating every non-sorcerer.
I see a lot of people saying that if Suguru had a proper therapy, he could have been "saved", but I can't agree with that, not entirely, at least.
Yes, he was in dire need of proper help after Riko's death, after Satoru's near death experience, he needed help to fight his identity crisis, but what he had needed way before that was a proper teacher and/or a parental figure, or a wider social circle, or a friend beside Satoru, or anyone, honestly, with a properly working moral compass who could have knocked those ideas of segregating sorcerers and non-sorcerers from each other out of his head. And, unfortunately, level-headed and responsible adults are what Jujutsu High and jujutsu society in general severely lack in (Nanami, forgive us, for we've fucked up).
It should have never been about "the strong protecting the weak", it should have been more about fighting the way one can to save lives, period. A sorcerer can't safely remove a tumor from a patient's brain to save their life, but a neurosurgeon can. A neurosurgeon can't exorcise a cursed spirit to save a life, but a sorcerer can. Both doing what they are able to in accordance with their innate qualities and abilities in order to achieve the same goal.
It's not about being better than others because one can do something other people cannot, it's about just doing what you can do. A pity there was no one to show Suguru that.
In the end, he was a child who'd fallen victim to adults' neglectful teachings and leniency, and got stuck in his simplistic beliefs. To me it feels like he'd never been given an opportunity for proper mental growth, and stagnated in his radical black-and-white (and naive, frankly) views and, eventually, his unhinged gargantuan dreams of making the world "better" in the most lunatic way possible.
And that's just sad. Not just the demise of a beloved character, but also the "what could have been".