I love Shouto, but me and Dabi had the same reaction to this line. Yes, Shouto, he has lost his mind. He just said that. Now please try to understand that this is not his fault.
Don't get me wrong - I 100% understand Shouto being mad here and not taking Dabi's side or really understanding it yet. And I definitely don't expect a teenager to get complex mental illnesses, either. But he didn't blame his mom back then, because even as a child he hated Endeavor and understood the pain he caused. He understood that ultimately, it was Endeavor that drove Rei to snap and pour boiling water on her son.
Surely, that is some kind of indicator, a parallel or foreshadowing, however you want to call it, that Shouto can also understand that Endeavor is the reason for Dabi's existence - obviously to a much bigger extent here, if only because Touya didn't receive help like Rei did (even though Endeavor's reasons for getting her help were wrong).
I know some people in this fandom will take Dabi's line right here,
were he more or less indicates to not even care about Natsuo, too literal, much like when he said he doesn't care about Twice or the rest or the League. (and if you are of that opinion, please don't interact with this post)
The thing is, mental illness is complex. And people on social media love to be woke and claim to be against the romanticization of mental illness, but when some of those people actually get confronted with depictions of the darker sides of mental illness, like with Dabi, they suddenly forget all about it.
Rather than just reading this as 'Oh, look, he doesn't care about anyone except for himself and his goal of destroying Endeavor' try to look past that at someone who died as a child, whose brain has been rewired during the important years of development that happen until you're an adult, and is now obsessed with one goal - the goal that somewhere in his mind seems like the one thing left for him to do to survive, to reclaim himself when his own father deemed him unworthy before neglecting him.
He has been driven to a point where he even risks the lives of those that were always there for him, those he truly loves, because to him it's either that or actually dying.
People deal with and react to trauma differently. And sometimes it's in an extreme and uncomfortable way like with Dabi, but rather than just making it about him, we should also focus on just how bad Endeavor's actions were. Some people were eating up that 'redemption' arc, yet this should show just how difficult it actually is for him to redeem himself. Not because someone killed people for the sake of ruining him, no - because he has caused for Touya to die mentally, pushing him to become Dabi instead.
The heroes seem to stand a chance after all now, with Best Jeanist and Mirio arriving at the scene, but are they really going to win if they will just stick to their old ways and fight these 'dark' victims instead of trying to save them?
Regardless of Dabi having been wrong about Jeanist, he was still right in saying that we need to be more critical of heroes. Which is why I hope, very strongly, that eventually, Shouto can come to see this and, just like with his mom, make it his goal to save someone in need, someone who has been let down, someone who much like himself has been a victim of Endeavor. And maybe at the very least Shouto can come to realize that a hero should try to save everyone, even those that are more uncomfortable to help.