" Snape’s Sectumsempra, which severed George’s ear, was originally meant for a Death Eater who was about to curse Lupin. Snape never liked or respected Lupin, but he’s always protected him. His use of Sectumsempra gives the impression that he’s returned to his Dark Magic ways, useful for convincing Death Eaters and the Order alike, but it’s likely the last time he ever casts a Dark spell or even one meant to attack. This night falls between the time he killed Dumbledore and the time he purged his entire being with remorse. "

― Lorrie Kim , Snape: A Definitive Reading