Worst. Teacher. Ever.
You know Severus Snape, even if you're new to Hogwarts.
You know that teacher that always gives you the stink eye, no matter how good you're being? The one who seems to live for making snide little comments in the corner of the paper you poured your heart and soul into, right next to the "D+"? The one who always calls on you when you don't know the answer, who bores his hateful little eyes right into the back of your skull anytime you open your mouth?
Picture that guy. Now give him the ability to turn you into a frog or poison you in your sleep. Ladies and gentlemen, meet Severus Snape.
Snape teaches potions and oversees Slytherin House…which makes him a shady character to begin with. Add to that his apparent interest in the darker side of magic and the fact that he really has it in for Harry, and you have an adversary that no eleven-year-old wizard wants to tangle with.
Consider his first interaction with Harry. Having been introduced to the Wizarding World and told that he's somehow world-famous—and that everyone loves him—he suddenly finds himself with a professor who cuts him no slack at all:
SNAPE: Clearly, fame isn't everything, is it, Mr. Potter?
This is Snape in a nutshell: he assumes that Harry is capitalizing on his fame when, in fact, Harry would rather be anything but famous. We see, also, a glimmer of jealousy in what Snape says—it seems as if Snape wishes that he were cresting the wave of fame.
A Herring, Not A Snake
Naturally, all of this antagonism (and pro-Slytherin propaganda) leads Harry to think that Snape's super evil and behind all of the dangers besetting Hogwarts. As it turns out, however, Snape ain't guilty as charged.
Oh sure, he hates Harry's guts for no apparent reason and he's clearly up to multiple flavors of sinister skullduggery, but hide the spirit of Voldemort? Not his bag, baby. Professor Quirrell—and Voldemort, Quirrell's own personal Man Behind the Curtain—says as much during the final confrontation with Harry.
QUIRRELL/VOLDEMORT: Yes, [Snape] does seem the type, doesn't he? Next to me, who would suspect p-p-poor s-stammering Professor Quirrell?
When even Voldemort recognizes how inherently evil Snape appears, you just know that he's a guy who seems capable of heinous crimes. (If only he'd trim that lank black mane…)
In literary terms, that makes Snape a red herring: a character who looks unduly suspicious in order to throw the scent off the real villain. And he does his job beautifully. For as much as he hates Harry, Harry hates him back… so much in fact, that he never bothers thinking that the real enemy could be someone else.
Of course, Snape's playing the long game just like Dumbledore is, and while he doesn't actually turn out to be the villain here, he's got seven more movies to make up for lost ground. We're pretty sure he hates Harry enough to make a really serious go at it.