lalala riot

Children tend to latch onto at least one of their parents pretty hard. In this case, Touya was a daddy's boy. He loved training, he loved the attention from his father, and he wanted to meet his father's expectations. It didn't matter that his quirk hurt him, because Enji was happy and it was "their thing".

And then suddenly he wasn't, and Touya wasn't allowed to train anymore. Yes, it was to prevent Touya from hurting himself anymore, but as a child, you don't see it that way. Dad has suddenly stopped doing "your thing", and on top of that suddenly wants more kids?

Children react to younger siblings in like one of two ways. Either they're super excited for the new sibling and want everything to do with them, or they hate them entirely, feeling that their parents have decided to replace them. They see this thing that can't do anything by itself and is taking all of their parents' attention away from them.

And that's where the Todorokis' failed. Enji was too busy trying to literally produce a child to be Touya's replacement and Rei was most likely dealing with her own issues as she was being used as a broodmare.

So yes. Touya was jealous. It's a normal thing in children. Shoto was the breaking point for him. Shoto was the one that his dad was most excited about, and Touya knew he'd finally been fully replaced.

Touya as a child was in extreme mental anguish due to basically being told that he isn't enough and never will be enough for his father, it's HONESTLY been clear the moment people realize Dabi was Touya that he obviously has mental instability caused by his upbringing and this just is the final revelation of that. A child lashes out like that because they're saying "I'm in pain- you hurt me- help me" and the only reason he attacks Shouto is because Endeavor has made an environment where Touya views Shouto as the threat that's taking away his father because he's a child, he can't see explicitly it's endeavor doing the hurting- thinks that it's shouto's fault so he lashes out toward them and after when he's stopped he clearly sits in these negative feelings of hurt and the negelct and no one helps him and so he tries to be "better" for his father "if I can just be better maybe my father will care" and ultimately this leads to his "Death" then he never comes home and is very likely a hurt and homeless child. And I mean clearly he realizes that as an adult it's endeavors fault and Endeavor was the one hurting him but he has his negative feelings of shouto that are associated with his childhood