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Severus Snape is a tragic villain in original Harry Potter series and redeemed himself. However, in some fanfictions, like Dumbledore's Army and the Year of Darkness, he serves as a pure-evil antagonist similiar with Lord Voldemort, Dolores Umbridge, Fenrir Greyback and Bellatrix Lestrange.
Dumbledore's Army and the Year of Darkness
As Headmaster, Snape inflicts his sadistic whims on the poor students and regularly blackmails the D.A. by punishing eleven-year olds in their place. He even ordered an eleven-year old flogged and left in chains for thirty hours. What Snape did to Michael as a punishment for freeing this kid was completely unforgivable: he had the poor guy tortured in a nightmarish way, prolonging the agony to the point where his best friend Terry tried to put him out of his misery with Avada Kedavra.
The author has been criticized for portraying Snape as way, way too evil.
Notebooks And Letters
In Notebooks And Letters, Severus Snape is also a pure-evil person. Although helping the trio and the others escape from Malfoy Manor, he refuses to do so until Harry drinks a specially concocted potion that will completely wipe out every pleasant memory Harry has, until he is ultimately left with nothing but his self loathing and a seething hatred for the entire world and all of those he once loved. Snape sees this as his ultimate revenge against James Potter, having refused to accept any responsibility for losing Lily, and as seen in a pensieve memory, relished that Harry would have to die against Voldemort and had asked Dumbledore if HE could be the one to kill Harry. When the group have escaped, Snape slaughters any survivors and burns the mansion down with Fiendfyre, and then kills himself so as not to face any repercussions.