I think Dabi's scars might come from overusing his quirk as a child. His fire might manifest from eyes and mouth in the same way Endeavor has fire eyebrows and mustache. But as a kid with his inability to control it during the grueling training his dad probably put him through it might have torn apart his face and arms. After which his dad abandoned him (as he did with his other children) and moved on to make another child with Rei. Thus causing Dabi to lash out, drop his name, and lose faith in his family. But thats just my theory (⊙…⊙ )
I think Dabi's scars might come from overusing his quirk as a child. His fire might manifest from eyes and mouth in the same way Endeavor has fire eyebrows and mustache. But as a kid with his inability to control it during the grueling training his dad probably put him through it might have torn apart his face and arms. After which his dad abandoned him (as he did with his other children) and moved on to make another child with Rei. Thus causing Dabi to lash out, drop his name, and lose faith in his family. But thats just my theory (⊙…⊙ )