hmm

A lot of people would like to point out that Fuyumi and Natsuo were neglected but grew up to be good natured people yet place Touya as the scapegoat for his own problems but you forget that they weren't lied to like Touya. The reason Natsuo didn't see anything wrong with being Neglected until he was an adult was because Endeavor didn't drill his obsession, ambition and beliefs like he did with Touya since he gave up on Natsuo immediately. The cruel thing about this is that Endeavor is aware that Touya knew this and hoped he would give up since he was replaced by Shouto and Touya knew as well that his father had replaced him.

Of course we know that Touya's motivation was always to gain his father's love and respect, he was the most important person in his life and of course his role model (unfortunately). He thought so highly of his father that he wanted to make him proud and prove he wasn't a failure, when in reality it was Enji who failed himself by trying to create an heir to surpass him and All Might in the first place.

The sad part for me is how Touya was in denial of his father being a bad person since he placed him on a high pedestal but even when he questioned it, he noticed no one in his family would listen and they would think he was the problem for pointing out Endeavor's flaws and provoking him since most of the family never wanted to admit the family was broken either out of fear for Endeavor or resigning themselves to their family's situation due to powerlessness.

His Father may have discarded him but his mother is also to blame as he saw right through her hypocrisy. Rei married Endeavor only for her family's sake and she of all people tells him he should not be bound by his bloodline. Telling him he should look beyond his father was good advice sadly Rei did nothing before and Toya was at the age where it was too late to do anything about it since he only saw her as a bystander who agreed to have more children to give Enji an heir to surpass All Might and replace him.

Rei also did nothing before to help relieve his emotional distress, from being a failed creation, since she was dealing with her own and when she did comfort a child it was Shoto, it was to relieve Shoto's pain since he was being physically abused through Endevor's "training". To Toya, Shoto was the masterpiece worthy of attention and care while he was just a failure who caused trouble for the family for trying to do what was his reason for even being born but told to quit while Endeavor never did.

And finally we have the incident on Sekoto Peak where Toya's tears became flames and he incinerated the entire mountain. He died because Endeavor never came since he was just a failed creation and thus not worth his father's time and even if Endeavor's reason was because he didn't know what to say to him he did nothing to for him as a father or even comfort him before and when Toya lashed out it became too much for him so he hired help and told Rei she should do it. Toya died knowing he was replaceable and discarded and the fact that Endeavor couldn't even bother to stop his goal despite his death means he meant nothing. It took Toya to almost die by burning alive on top a mountain where he used to be happy training with his father to realize his family never cared for him. The same father and well renowned Pro Hero Endeavor couldn't even come to see his own son...

HELL He couldn't even save him from being burned to death or ever taught him how to regulate his flames because Toya wasn't worth his time. He's a hero and his job, career, and ambitions will always be more important and because he's a HERO he can get away with discarding him after he outlived his usefulness. On that day when he burned a top Sekoto Peak Touya died...and so did his love for his father....and his family.

Unbeknownst to anyone Toya survived albeit with horrible burns, and managed to leave the accident without anyone noticing him.(imo he must have had help from Ujiko)

After that incident he would not let his pain and suffering go unnoticed and knew that he couldn't go back home not that that place was a home to begin with.

On that faithful day he burned and rose from the ashes and was reborn as a Villain with a newfound purpose in life and one his father will surely look at.

Endeavor's greatest crime was best foreshadowed here in chapter 85 from Shigaraki explaining to Bakugo the crimes of hero society and the panel that has Dabi during Tomura's rant:

Endeavor's greatest crime was abusing his family, creating a family for his sole ambition, Endeavor never did anything as a father for his children or as a husband. He dodged responsibility and it resulted in his eldest son dying.