Both parents are at fault. Enji made Touya believe he could surpass All Might, he made him work hard and basically told him he´d only be proud and acknowledged by him if Touya worked hard enough- and became his successor.
However, Touya took this so serious that he felt the need to hurt himself in the process, cuz he simply did not care about his health- what he cared about was his father recognition (Which is understandable for a 10 year to do, considering the fact he´s literally been training and pushing himself ever since he was born)
Yet, when Shouto was born, and he saw how his father didn´t pay attention anymore- He felt betrayed, as if his whole life had been a lie and everything he had done was for nothing. All the practising, all the sleepless nights, all the hurt- Everything had gone to waste, and worst of all, his father even told him that it´s not neccesary to train anymore (In Touya´s ears it must´ve been horrible, cuz he basically told Touya he didn´t need him anymore, as if he was just a broken tool)
Touya is in no way innocent. But the way he was treated by both Enji and Rei is what drove him to be Dabi. It´s what drove him to attack baby shouto and It´s what drove him to kill off Touya itself.
His actions, are Enji and Rei´s creations.
Nothing more, nothing less.
frick endeavor man he messed up all his kids