" This is the answer to Sirius’s question about why Dumbledore would hire someone who once worked for Voldemort. People can change, even if their spots cannot. Appearances and inner reality are not always the same. There are sometimes good reasons to present the self untruthfully, even if that can be a grueling ordeal, as Sirius well knows. And people who have undergone such transformations know things, can do things, that those innocent of such ordeals cannot. "

― Lorrie Kim , Snape: A Definitive Reading