Good vs. Evil Quotes in Harry Potter and the Sorcerer's Stone
How we cite our quotes: (chapter.paragraph)
Unfortunately, you needed a specially signed note from one of the teachers to look in any of the restricted books, and he knew he'd never get one. These were the books containing powerful Dark Magic never taught at Hogwarts, and only read by older students studying advanced Defense Against the Dark Arts. (12.33)
Like Ollivander's wands, these books have the potential to do evil, but they're not evil on their own. Their presence is necessary, in fact, for those who wish to learn about protecting themselves against Dark Magic. It makes sense that these books should be "restricted" from first-year students, but as Harry's actions show, rules and restrictions can only go so far in keeping people from learning the things they want to know.
Harry knew Ron and Hermione were thinking the same as he was. If Snape had been in on protecting the Stone, it must have been easy to find out how the other teachers had guarded it. He probably knew everything – except, it seemed, Quirrell's spell and how to get past Fluffy. (14.39)
In this context, Snape seems totally evil. Harry, Ron, and Hermione have a certain amount of evidence, and they have guessed rightly in many aspects of it. One of the teachers "protecting the Stone" is in the best position for figuring out how to get at it later. They're just wrong about which teacher is secretly evil, at least in this case.
"That is because it is a monstrous thing, to slay a unicorn," said Firenze. "Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips." (15.155)
Just in case we were unclear about the question of why killing unicorns is bad, Firenze reminds us with a flurry of strong adjectives: "monstrous," "terrible," and "cursed." Of all the crimes out there, this is one of the worst, only "commit[ted]" by those with "nothing to lose." Committing this crime carries with it its own instantaneous, permanent punishment of a "half-life."