Professor McGonagall's voice trembled as she went on. "That's not all. They're saying he tried to kill the Potter's son, Harry. But – he couldn't. He couldn't kill that little boy. No one knows why, or how, but they're saying that when he couldn't kill Harry Potter, Voldemort's power somehow broke – and that's why he's gone." (1.72)
The book's barely started and already good has triumphed over evil; what's more, the good in question is a mere baby. When faced with the goodness of that little boy, the evil wizard's "power somehow broke," even though he had time, experience, training, and power on his side. None of those were a match for the unformed goodness of this baby.
"Better Hufflepuff than Slytherin," said Hagrid darkly. "There's not a single witch or wizard who went bad who wasn't in Slytherin. You-Know-Who was one." (5.199)
Well, this is kind of rough on any Slytherins without homicidal tendencies, isn't it? Seriously, though, Hagrid's statement raises questions about how much of good and evil can be put down to fate, and how much to personality. What would have happened if these "bad" wizards hadn't been put in Slytherin when they were at Hogwarts? Would they have ended up as good, responsible citizens? For more on this, see our section on "The Sorting Hat" in "Symbols, Imagery, Allegory."
"I'm sorry to say I sold the wand that did it," he said softly. "Thirteen-and-a-half inches. Yew. Powerful wand, very powerful, and in the wrong hands…well, if I'd known what that wand was going out into the world to do…." (5.225)
Ollivander makes many, many wands, and most of those are used for good. He has no way of knowing, when he sells them, which wands will be used for more evil purposes. But the wands he makes, in the wrong hands, can kill people and cause terrible wrongs. Does that make him responsible at all for the spells some of these wands' owners perform?
"I've heard of his family," said Ron darkly. "They were some of the first to come back to our side after You-Know-Who disappeared. Said they'd been bewitched. My dad doesn't believe it. He says Malfoy's father didn't need an excuse to go over to the Dark Side." (6.262)
Somewhat conveniently, Malfoy's family blames their alliance on the "Dark Side" on being enchanted themselves. It's the kind of defense that's impossible to prove or disprove, and makes them suspect to people like Mr. Weasley, who believe that the Malfoys were already inclined to Darkness.
"No – he wouldn't," she said. "I know he's not very nice, but he wouldn't try and steal something Dumbledore was keeping safe."
"Honestly, Hermione, you think all teachers are saints or something," snapped Ron. "I'm with Harry. I wouldn't put anything past Snape. But what's he after? What's that dog guarding?" (11.29-30)
According to the conclusion of Book 1, Hermione is right: Snape isn't on the evil side here. While Snape acts really weird and suspicious throughout the book, giving Harry and Ron plenty of reasons not to trust him, ultimately Dumbledore himself defends the potions master. It would appear, then, that Hermione is right in her separation of Snape being "not very nice" from the idea of him working for the Dark Lord.
"Can't have," Hagrid said, his voice shaking. "Can't nothing interfere with a broomstick except powerful Dark magic – no kid could do that to a Nimbus Two Thousand." (11.105)
Many factors combine here to emphasize how scary the enchantment of Harry's broomstick is. One, whatever did it is something that scares the large, confident Hagrid, who describes the situation with "his voice shaking." Two, broomsticks are complicated objects on their own and it would take some extremely "powerful Dark magic" to mess with them. Three, whatever did this is "no kid" but an adult with full-fledged magic powers.
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."
Lord Voldemort showed me how wrong I was. There is no good and evil, there is only power, and those too weak to seek it… (17.31)
Spoken, of course, by someone on the "evil" side! If you've ever wondered how bad guys justify being bad, this statement by Quirrell is a fine starting point. To keep himself from doubting the "goodness" of his actions, Quirrell thinks only in terms of "power" and its absence. Of course, the absence of that power is defined by "weak[ness]"; having and manipulating power has positive connotations, while not exercising it – for whatever reason – has negative connotations.