In the first episode of the season, after saving Utahime, Gojo teases her by saying "you cryin'?". Geto enters the conversation and his first line is "Satoru. It's not nice to pick on the weak." Meimei points out that Geto is the one teasing her without knowing it. This is a hint to Geto's character: someone who naturally dislikes the weak yet he cannot see this truth due to him following the obligations as a jujutsu sorcerer to protect the weak.
When they saved Utahime, Gojo forgot to put up a barrier. This leads to him being lectured by Yaga. After, Gojo asks if curtains are even necessary in the first place. Geto answers by saying "of course it matters. The prevention of cursed spirits is most important for the citizens' peace of mind. For that reason, keeping it confidential is important. Not only that-" and then he is cut off by Gojo. This shows that Geto is by the book and that they have already had this conversation before as Gojo cuts him off as if he already knows what Geto is about to say. This establishes that Geto is the level-headed and ethically just counterpart to Gojo, who is reckless and selfish.
While protecting Riko, Gojo wasn't as strong as he is now. He hadn't learned the reverse cursed technique, keeping his infinity active at all times would strain his body, and he couldn't use red or purple. When Geto realized Gojo hadn't slept for 3 days, Gojo said "I'm good, man. Besides, you're here too" and this makes it obvious that he felt safe with Geto. Gojo often fought with Geto, trying to assert his dominance, but Geto did the exact same. They used to be the strongest duo together.
The assassination of Riko made both of them emotionally distraught. After Gojo kills Toji and carries Riko's corpse out of the chamber, Gojo asks "Should we kill these guys? If we do it now I probably won't feel a single thing". To which Geto responds by saying "Nah. There's no meaning to doing that." Gojo then responds by saying " Meaning... Is that really necessary?" Geto answers "Yes, it's very important. Especially for sorcerers." At this moment, Geto is able to maintain his composure and convinces Gojo to not massacre these cult members for no reason. By doing this, Geto truly shows the extreme to which he would go to follow his duty as a jujutsu sorcerer.
As the morally ethically just side of Geto is extreme in the way that he follows his duties is one side of Geto. The other side is also one that Geto followed to an extreme.
Toji gave Gojo his blessing (Megumi in japanese means blessing) and gave Geto his curse (the curse Toji used to store his items) not only physically but also mentally. Gojo became stronger thanks to Toji while Geto could only watch Gojo becoming stronger and more distant because of Toji.
Gojo becoming stronger separated them and made both of them lonely. By seeing Gojo as "the strongest" instead of them both being "the strongest", Geto falls into isolation and although Gojo doesn't show it as much, he also fell into isolation. Geto now no longer has a friend who's there to stop his downward spiral.
Seeing the cult members mindlessly clap over his friend's death at Geto's highest point of grief took a big part in Geto's coming to his realization. After being traumatized by the claps of the cult members, Geto is at his limit. He asks himself "For who?". He wonders for who he is suffering. Geto says "Ever since that day... I've kept asking myself. What I saw then wasn't unusual at all. I was well aware of that ugliness. I already knew that and I made the decision to be a sorcerer despite it. As one who is strong, you must fulfill your obligation". After being reminded of the clapping, he finally breaks by saying "You fucking monkeys".
When his best friend betrayed him, Gojo felt insecure and lonely and he realized it wasn't enough for him to be strong, he needed strong allies to back him up, which is why he became a teacher.
Haibara's death deeply affects him because we could see that he acted as one of Geto's emotional pillars. Geto doesn't want to see any more of his friends dying, which is the main reason for him killing all non-sorcerers. He doesn't want to see his friends die for the sake of the same people who mocked his friend's death. In jujutsu kaisen 0, Geto still sees all the sorcerers as his friends
I could also talk about Yuki accepting Geto's plan and in a way encouraging him by saying that it's the easiest way to achieve her dream even though what he needed most was somebody stopping him, Geto deciding to finally take action after seeing two young girls tortured by the village and many others but I don't want to repeat too much of what others have already said