Many at Hogwarts have reason to fear Severus Snape, a reformed Death Eater now teaching at the school, who skulks around, sneering and hissing at Harry Potter. Even though Harry and his friends despise Snape, Dumbledore continues to trust him implicitly, dismissing Harry’s stories about overhearing Snape plotting with Draco and insisting that Snape is completely trustworthy, an irreplaceable member of the Hogwarts staff. Harry’s father, James Potter, was also at odds with Snape, and members of the Order of the Phoenix suggest that Harry’s inherent dislike of Snape was inherited from James. Harry is appropriately devastated when he learns that Snape and his beloved Half-Blood Prince are one and the same.
Although Harry sees Snape’s shiftiness as black and white, Rowling is purposefully vague about Snape’s true allegiances. Although Harry Potter and the Half-Blood Prince features a pointed scene in which Snape makes an Unbreakable Vow to Narcissa Malfoy, promising to protect Draco and take over his Dark duties, if necessary, there are still signs that point to Snape’s allegiance to Dumbledore. Moments before Dumbledore is murdered, Dumbledore freezes Harry, presumably to prevent him from stopping Snape. Likewise, Dumbledore appears to be stalling Draco until Snape can arrive, after specifically requesting that Harry call for Snape and not Madam Pomfrey— making it seem possible that Dumbledore wishes for Snape to be his killer. When Harry chases after Snape, Snape prevents Harry from casting any death spells, but once again does not try to kill Harry himself. Snape is an extremely complex and multi-faceted character.