Harry Potter just looked at Professor Snape, with rapt attention, although he was rapidly considering that Snape's knowledge of the Dark Arts was something like an abyss - it was possible to get into it, and barely possible to get out, but it was very difficult.
"Truly black magic is created out of intent. Hatred, primarily, although the truly evil can create vile magic out of unadultered sadism. Chief example among which is the Cruciatus Curse," Snape began*, and Harry found himself thinking of Bellatrix Black, her face twisted into a caricature, loathing and repulsion at the fore. "The Ministry of Magic, being a bureaucratic institution, has decided to lump all magic it deems dangerous, unproductive, or is merely afraid of, as Dark Arts."
"True Dark Arts are a different matter entirely." Snape said, "They arise out of helplessness, hopelessness, uselessness. You might term it a survival instinct. Unlike black magic, this is pure resistance to being conquered. At the last, when everything and everyone has failed you, your mind turns to the Dark - and conjures a darkling light."
Harry Potter tried to comprehend this, his mind taking him back to his cupboard under the stairs. He asked himself what he would have done if they'd locked him in, and thrown away the key. Certainly he'd wondered about that very thing, often enough.
"Dark Arts, without exception, are more powerful than what an individual can normally accomplish. They are also highly individualized, and resist systematization with a will of their own." Snape said.
Harry Potter asked the obvious question, but with a good deal more trepidation than normal, "If they're so powerful, why doesn't everyone learn them?"
"First, they're a crutch and a cheat. Nearly everyone will be stronger by taking the long road of building their knowledge and power the ordinary way. Some, like Miss Granger, will be more powerful in the short term, even." Snape said, seemingly unaware that he'd complemented a Muggleborn. His tone was clinical, and it was merely an observation, even if it surprised Harry to hear it from Snape's mouth.
"Second, they're addictive. If the only way you have to survive is to train yourself into that peculiar brand of hopeless, helpless fulminating rage... It starts to become something you seek out. In war, that will, one hundred percent of the time, get a person killed. In peacetime, it is scarcely better." Snape said sternly.
"Thirdly, to push someone, shape them into a mindset where they can use the Dark Arts, is to break them, in mind and soul. Not like using a mace or a sword, this is corrosive, because the Dark Arts run on a sense of betrayal that runs bone deep." Snape said. "I will not teach any of my students the Dark Arts, not if I can find any way around it."
Harry's lips thinned into a line that was the precursor to a smile that never materialized. He had learned, this past summer, the many twists of a Slytherin mind. A determination like that was not easily misled or bamboozled. "Sir, you said that the Dark Arts are unreliable...?"
"I lied." Snape said smoothly, "And I will continue to lie, to prevent students from deliberately trying to maim themselves or their friends in pursuit of power."
"So, why me?" Harry Potter asked directly, trying to understand what was going on, "I'm no mindhealer..."
"No, you aren't. But you've been through enough to want to help these students, and that may be their only saving grace." Snape said, starting to pace, "Make no mistake, I meant precisely what I said in class - I could have you casting Dark Arts spells reliably, within a day." Snape's face tried to smile, but it came out looking far more like a snarl, "It would be easy."
"And yet you say that Neville..." Harry started, timidly, considering his every word.
"Neville Longbottom holds his rage deeply. He's not practiced in quenching it - he nourishes it, it impels him to great deeds." Snape said, looking down at Harry Potter. "You're not angry right now, are you?"
"No," Harry said, tasting the word as it fell from his lips. He really wasn't, he thought, he was getting answers, and this lesson was kinda interesting. Horrible, but interesting.
"That's why young Draco Malfoy tweaks your tail, you know. It's nearly safe - you get upset, he has a laugh, and twenty minutes later it's all water under the bridge." Snape said, "Oh, you don't like him, but you've never liked him, so what harm's been done?"
"If he tried the same with Longbottom..." Snape let his words trail off, and Harry Potter saw an image of Neville's meaty fists beating Malfoy into a pulp, "but he wouldn't, he knows better." Snape's lips quirked, for a moment, as he said, "He's even learnt his lesson with Miss Granger, I believe."
"He hexed her in class!" Harry Potter shouted, seeming more astounded than outraged.
"Testing, always testing." Snape said with a firm basilisk stare. "If he had wanted to choose something more... damaging, he could have, I assure you."
Harry Potter resolutely turned the topic away from Malfoy, whom he really didn't want to be discussing with the notoriously biased Head of Slytherin House. Particularly when said head was likely to have a smooth answer to whatever rough question Harry could formulate. "Have you ever mastered the Dark Arts?" Harry asked, although he already knew the answer. It was a feed, a request for more information.
*Yes, when Snape was training Harry, he was primarily drawing on sadism to do so. Some people could cast a spell to torture someone with pure intent (zealots primarily - 'I will cleanse your soul'), Snape's not one of them, nor has he ever claimed to be.
[a/n: Dark Arts spells in this AU are extremely individualized. The ministry just put everything in one bin, and threw it away.
Now you know, at least part of the reason, why Snape decided to play Poke the Potter where it hurts.