Severus Snape as an Anti-Hero:
Snape displays many characteristics of being an anti-hero rather than a villain. He can also be associated as a Byronic Hero. A Byronic hero, unlike the villain is a type of anti-hero that originated during the romantic period. Typically characterised by an antisocial attitude, is usually dark, passionate, intelligent. Snape at many occasions has demonstrated his choice to truly belong to the good side still however he will never be qualified as a hero.
He is difficult and unpleasant, and has no qualms to subject his students to this attitude. Snape was once a Death Eater and therefore implying his loyalty towards Lord Voldemort (Goblet of Fire). We know very little about the early life of Snape making it easy to believe that Voldemort and his ideologies were something Snape greatly admired, (“Snape knew more curses when he arrived to school then half of the kids in seventh year”, according to Sirius Black). Snape had also invented a curse , Sectumsempra (Half Blood Prince), which is rare for a student. He demonstrated no qualms to use the word “mudblood” (a foul word used for the muggle born or the impure) in the most derogatory sense (as Harry witness during his visit into Snape’s worst memory in the Order of the Phoenix). He also fancied a nickname for himself, The Half Blood Prince, highlighting his wizarding heritage.
These small facts, taken together, are indicative to the reasons Snape may have chosen to become a Death Eater. His words