One of the most depressing backstories in the entire series of My Hero Academia belongs to Shoto Todoroki. Because of his father, Shoto was forced to endure torturous training and never-ending family strife; how he handles his traumatic upbringing defines him. A bright red scar that spans half of his face is another reminder of his past that he is forced to carry with him. Everyone already knows that Shoto's mother burned him with hot water when he was a child because Endeavor's eyes reminded her of his.
The thin and sensitive skin over the face makes boiling hot water more than sufficient to leave a scar. It appears too vividly colored, though, for boiled water to be the only culprit. Given that Shoto controls fire with his left side, it seems improbable that boiling water could have left such a mark on that particular side of his face. It turns out that the scar that has come to symbolize Shoto's identity actually has an even more tragic meaning.
Rei Regretted Pouring the Hot Water on Shoto Right Away
After Rei dumped boiling water on Shoto, she immediately felt guilty and cried out, "I'm so sorry, Shoto! What have I done?!" and promptly used her ice Quirk to cool the burning in the middle of all the panic and chaos. Unfortunately, this just made it worse. Treating a burn with ice will have adverse effects since it causes instant, irreversible damage to skin cells, resulting in a frost burn. Ultimately, Shoto may not have been scarred from the hot water alone and Rei using her ice powers is what caused the scar, or at least made it a lot worse.
Applying ice to a burn is a common error that many people make, and first aid for burns is somewhat counterintuitive. It would have been safer for Rei to take Shoto to the hospital after the boiling water made contact with his skin given the tense nature of the situation. Rei immediately panicked and attempted to calm it down with her powers, but given the situation in the Todoroki household at the time, she ultimately made things worse for Todoroki. This fact only adds to the sadness of Shoto's scar.
Heat-Resistant Left Side of Shoto
Given the nature of Shoto's quirk, the boiling water probably wouldn't have left a scar had Rei not used her ice powers. Since the left side of his body controls his fire, Shoto is more resistant to heat on that side. Though it was undoubtedly stressful and scary for a young boy to endure his mother dumping water on his face, but he probably would have come out physically unscathed -- if Rei had simply remained calm.
The unfortunate truth behind Shoto's scar is that he would have been fine if the incident had been handled differently. Unfortunately, Rei allowed her emotions to take over during an already tense situation, further traumatizing both herself and her son. The most heartbreaking aspect of the situation is that Shoto now has a visible reminder of his childhood trauma on his face. The sixth season of My Hero Academia focuses on the steps Rei and the rest of her family are taking to mend their relationship with Endeavor, hopefully recovering from his abuse and putting an end to Toya's rampage.