"Bellatrix Black was perhaps the only one of the original cadre of Death Eaters - Knights of Walpurgis that was in any way admirable." Snape said, consideringly, his eyes unfocused, seeming to look into the past.
"Riddle had charm, certainly, and Goyle and Crabbe strength - and Lucius, even then, had a unctuous manner that bent truth as easily as an eagle steals fish." Snape shook his head, "Bella, though - she saw truth through those flinty eyes of hers. And where she saw it, she'd glow like a fire. Righteous and Vengeful at the same time." Snape shook his head again, "I miss her."
Harry stood respectfully silent, his fingers playing against one another behind his robe. Snape was speaking as if the person he knew was dead. And, perhaps, she really was. Certainly, Harry Potter had never seen her like that - tall and strong. Through Snape's words, he could almost picture it, mentally redrawing the mentally deranged person he'd met, as someone who'd been... more.
Harry's thoughts twisted, juked over to Sirius - what, really, had he been like, before Azkaban? Oh, he'd seen Snape's memory, and sure, that was at least a part of him. Lupin hadn't - but Lupin wouldn't, perhaps couldn't, look on his old, dear friend as if he was a shambolic wreck.
"Dangle just a bit of truth to the masses, and let them learn the hard way." Snape said, consideringly, "Yet another reason for pointing you out. Give them the idea that someone can learn Dark Arts - dangle the possibility in front of them."
"Why would you do that, if you don't want them to learn them, sir?" Harry Potter asked, confused.
"Because they might learn it by accident," Snape snapped, his tone harsh but his eyes looking offscreen. Harry wondered if he'd learned the Dark Arts by accident, and then mentally shook his head at himself. Of course he had. James and Sirius wouldn't have intentionally given him power.
"You care, you realize?" Snape said, shaking his head, "Were someone damaged to land on your doorstep - I don't think you have it in you to turn away."
Without nodding, without moving a muscle, Harry Potter stood there, wondering whether that was a Good Thing or not. Hesitantly, he decided it was both.
"Drawing someone back from the edge, pulling them out of the abyss - it's not nearly as hard as you think." Snape said, spinning on his heel, offering over his shoulder, "But it's impossible if you don't care." Snape turned around, spreading his hands, "Someone who's given their heart, their mind, even their soul, to desolation - they have a million ways to slip through someone's fingers." Snape's eyebrows rose slightly, "It takes determination, persistence, even patience, to win them back."
Snape paused, taking a deep breath, quirking an eyebrow as Harry was silent still. "You might wonder why the Dark Lord (aptly named, he of the perpetual ace up his sleeve) doesn't deliberately break his servants, and then reforge them." Snape shook his head, "It's a fate that one who's already shared it wouldn't wish on anyone. Better to be as mindless as the Longbottoms, surely."
Snape let those words hang in the air, and Harry listened, his mind paging back to seeing them in St. Mungos'. They had looked so fragile, so frail - and yet so ... alive. Not like their souls were gone, not shapeless husks. Just...mindless. Staring blankly into the distance, until you almost swore they'd forgot to blink.
[a/n: yes, this has become an exercise in "how many reasons"... sorry if you don't like that sort of thing.
Yes, if Harry wanted, he might be able to take this knowledge and become a New Dark Lord.
Why do you think Snape's telling Harry, and not someone else?]