" With close calls, as long as nobody dies, the attacker can walk away, free to resolve that they will not repeat the mistake. But sometimes attacks don’t end well and there is no walking away from the aftermath. Harry was immediately aghast at the effects of Sectumsempra, showing that guilt from causing damage or death can be enormous, even if the attacker didn’t know what they were doing. "

― Lorrie Kim , Snape: A Definitive Reading