Although Harry has a mostly positive introduction to Hogwarts, Professor Snape makes it clear from the very first night that he does not like Harry at all. In fact, Snape does not like most of his students. Professor Snape is the Head of Slytherin House and shows favoritism to the Slytherins. He is quick to mock and punish his students, particularly the Gryffindors, if they do not immediately get their potions correct. He dresses in dark clothes, teaches in the dungeons, and is known around Hogwarts for being almost constantly in a bad mood.
Harry collects clues that convince him Snape is helping Voldemort. For example, Snape is bitten by the three-headed dog while trying to sneak past it on Halloween. Hermione sees Snape seemingly performing a spell on Harry during his Quidditch match and assumes Snape is cursing Harry’s broom. Harry overhears Snape threatening Quirrell. All these clues turn out to be red herrings, though. Because Harry knows Snape dislikes most of the students, especially himself, he assumes Snape must have bad intentions behind all his actions. In reality, Snape tries to stop Voldemort’s return throughout the novel and actively protects Harry from Voldemort’s advances. Professor Snape is unpleasant, but not evil.