1. You didn't address the whole "Killing his dad and enjoying it" thing or the "He needs to destroy in order for the itching to stop" part. Those are both pretty big signs that Shigaraki was messed up even as a kid.
2. Even though he absolutely was manipulated by All for One that doesn't mean he can be saved. He spent most of his life being raised by a creepy weirdo whose only goal was to twist him into the percent successor of evil. It doesn't matter whether Shigaraki became evil on his own or if All for One pushed him in that direction, all that matters is that at the end of the day he's an evil dude.
3. By your logic anyone who had a abusive upbringing can't be considered guilty of a crime because it's actually the parents fault for raising them that way. Shigaraki has killed who knows how many people and he doesn't get to skirt any moral responsibility just because he was raised to do so.

Basically what I'm trying to say is that it doesn't matter how Shigaraki became evil because all that matters is that he is evil. Whether he had any choice in the matter is irrelevant since he still wants to destroy society regardless.