When you are a child things always feel bigger than what they are.
I'm an adult and if someone started to yell at me saying I should kill myself well, I would feel scared and bad about it, but I have coping mecanisms I can rationalize it and say (and feel) "the problem is not me, the responsability lies in that person".
A child on the other hand doesn't have that, it takes that experience and makes it theirs, it gets processed differently, to my understanding Natsuo, just like Fuyumi, was neglected (a type of abuse too) while Touya was actively getting physically and mentally abused.
In Touya's case I think he broke from all the kind of attention he received, first he was the golden child, first born with a good quirk with his parents being proud of him, then he was defective, his quirk hurt him and he kept getting pushed, then he was set aside and replaced, his little brother had the perfect quirk, there's a reason parenting books always talk about discipline being consistent.
Though as I have always said, after you are 18 it's your own responsability to take charge of your own mental health but I understand depending on the case it could be very complicated I feel like we are still missing a lot information about Enji and Touya and wonder if we'll get it.
I should probably stop harping on Dabi but his behavior just astounds me. He has spent more of his life away from Endeavour than actually with him and the beginning years of life don't even appear to be bad yet he's pretty cold blooded.
Natsuo is walking around feeling upset about his brother's death. Even after all these years the pain is still there. Meanwhile, Dabi sends a villain out that nearly kills Natsuo and his reaction to nearly losing Natsuo is basically like - I could care less if Natsuo died. Natsuo - the same brother that he played together with and the same brother that comforted him when he was upset. This is an interesting look at how each views the other.
Dabi said the past never forgets . . . But what about the past with his brothers and sister. The past certainly seems to have forgotten the love he received from them. I guess Dabi's past has selective amnesia when it's convenient .