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Quirinus Quirrell (Ian Hart) Harry Potter and the Sorcerer's Stone
Why? Jinxed Harry's broomstick during a Quidditch match; was out to steal the Sorcerer's Stone; was the host body for Voldemort.
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Gilderoy Lockhart (Kenneth Branagh) Harry Potter and the Chamber of Secrets
Why? Placed fame over teaching; mistakenly removed the bones in Harry's arm while attempting to heal him; tried to erase Ron's and Harry's memories.
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Remus Lupin (David Thewlis) Harry Potter and the Prisoner of Azkaban
Why? Taught Harry the Patronus spell; led the class in battling Boggarts; revealed the secrets of the Marauder's Map; turned out to be a werewolf.
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Mad-Eye Moody (Brendan Gleeson Harry Potter and the Goblet of Fire
Why? Was actually a guy named Barty Crouch Jr., who impersonated famous Auror Moody while the real Moody was held captive; helped deliver Harry to Voldemort.
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Dolores Umbridge (Imelda Staunton) Harry Potter and the Order of the Phoenix
Why? Didn't allow students to practice spells; forced Harry to write, ''I must not tell lies,'' words that were magically and painfully etched on his hand.
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Severus Snape (Alan Rickman) Harry Potter and the Half-Blood Prince
Why? Was suspicious of Harry's sudden skill in Potions class; murdered Dumbledore (his true motives yet to be revealed).