Theory: Toji was Naobito's favorite nephew

I get that we barely saw this two interacting but I noticed that for all his jerkiness, Naobito was surprisingly reliable in matters regarding Toji. For starters, despite Toji's hate for his family, he apparently trusted Naobito enough to sell Megumi to him while privately hoping he would get a fair treatment and considering his uncle allowed Gojo to take care of his son despite his hate for his clan and more importantly eventually making him his heir apparent (conditionally but still) he was apparently right on the money. Combined with his giving him Toji extra money with a smile on his face, his being affected by seeing his zombie, their mutual smiles and perfect synch when doing a combo against Dagon and his personal dislike of the man who killed his nephew, it all gives the impression that Naobito had a soft spot for Toji. This makes even more sense when you realize the two are really similar in both personality and fighting style, not to mention the (albeit superficial) respect Naobito's son showed for Toji.

My take on this is that Naobito was the closest thing a loving father figure, Toji ever had. Say, despite being held in high regard because of his talent as a sorcerer, Naobito had never been all that fond of his more stuffy hardass traditionalist family, having a more relaxed personality coupled with the love for modern things like cameras, videos and animation that he based his technique on (which we know from Hakari traditional jujutsu users frown upon), which he in turn kept to himself in order not to affect his standing in the clan. Then his older brother had a second son with zero cursed energy, which he couldn't help but sympathize with, who eventually started showing similar signs of dislike for the stuffiness of his clan, causing the sympathy to grow into fondness as he took the boy under his wing. Toji's penchant for confusing high speed movements that he showed against Gojo is something he learned from Naobito, who tended to pick him as a partner when going to missions and his hedonistic tendencies come from following his uncle's fine example and he became a mercenary from his habit of doing jobs for his uncle in exchange for the "treats" he tended to give him.

However, their closeness would've probably been superficial as its well established that Naobito is not the sort of man to go against authority when he sees something unfair happening, meaning that in the face of the abuse the rest of the family put Toji through he barely did anything to protect his nephew. With time the treats his uncle gave him became a reminder of the standing he might've otherwise had in the clan, which eventually lead to a decline in their relationship as Naobito had his sons, to the point where a child Naoya hadn't met him. Eventually Toji had his rampage killing several of his abusers and sparing the rest out of whim according to Ranta, which I'm guessing was because of the kindness Naobito had showed him.

I think the realization that he had never trully helped his nephew in meaningful ways is part of the reason why Naobito, who at his best was never exactly a responsible guy, became his jerky current self which in turn contributed to the fithy piece of work Naoya turned out, having mimicked the man whom his distant father had always wished was his son. While he at first saw his taking in Megumi as a way to do right by Toji (while giving one last treat to his nephew), the news of the latter's death affected him deeply, comming to the conclusion the boy was better off as far away from him as possible, allowing Gojo to take him in knowing that it would eventually facilitate the boy helping make peace between their two clans, which he was unwilling to do with the man who killed Toji, for which he made him heir in case something happened to that man.

I also suppose his making things more difficult for Maki was his weird petty way of comparing her to Toji (*if he didn’t get any help why should she?*) but at the same time his saving her throughout the fight with Dagon implies he might've indeed been fond of her in his own way due to her reminding him of his dear nephew. Maybe that was his way of pushing her away from the clan not wanting her to end up like Toji, who in end still wished his family had accepted him.

Going with jujutsu Kaisen’s theme of competent outsiders putting the effort to help fix the world‘s bullshit, you could say Naobito is a successful outsider who is simply not willing to help bring this about, even if he wishes he did and hopes the next generation will do so.