Harry learns from Percy, even at his first night at Hogwarts, that Snape "knows an awful lot about the Dark Arts." 4 Sirius confirms in Harry Potter and the Goblet of Fire that "Snape's always ...
Snape's wounds never heal, causing him to lash out. Unfortunately for Harry's beloved godfather, Sirius Black, this leads to tragedy. In Order of the Phoenix, Sirius is sequestered in his family home.
At this point, it does not seem likely that we will see Snape forgiving James and Sirius for their treatment of him at school any time soon. Grudges that are held as long as Snape's has been are ...