The Middle Distance

a. He was the first of someone else's characters that I really devoted myself to writing, so my desire to write Snape is inseparable from the history of my desire to write fanfic at all. I think fandom itself helped me love him; here is someone who's nasty, bitter, and canonically rather ugly (this is important!) and yet with that je ne sais quoi. I've loved/hated (mostly loved) him since we first meet him in the books.
b. Ruthless intelligence.
c.. Bitterness. Utter poison, that. I'm sure he's good with poisons because he's done it to himself for so long.
d. Difficult and easy at once - his voice comes fairly naturally to me, but I always wonder if my infatuation makes him too nice. I imagine him reading my accounts of him and rolling his eyes and being utterly vicious about my writing talent or lack thereof, and that helps me get it closer to right. :)
e. Again, I don't know if I ever have. But I still like this exchange between him and Hermione (whom he's shagging, passionately and thoroughly unethically), after she's just killed someone with Dark magic for the first time:

"When you...back when...when you were..."

"When I was a Death Eater?" He said it impatiently.

"Yes. Did you...?"

"Yes."

"It was a long time ago," she said. "You were a different person then."

"Yes and no."

"Have you ever felt...like you were back to sort of what you were like before it all happened?"

"Of course not, that's absurd. You can't ever take back an experience."

Her glass was empty, and he refilled it, nearly wary of her hungry eyes.

"Did you ever...get over it?"

"I haven't the faintest idea what that means," he said coolly. "Please clarify your terms."

"Get over it," she said, deeply gulping wine. "I mean get free of it. To feel normal again, like there's not some kind of monster festering inside, that every time you pick up a wand you're not capable of....anything."

"But you are," he said evenly. "Subject to the laws of magic and limits of your own power of course."

"That's not what I mean! I mean - do you ever feel clean?"

He laughed as he refilled her glass. "Oh, that's what you mean? No, Miss Granger, honestly I don't, ever, and I never have. Clean? No. Souls aren't - they're dirty complicated things. But I don't think it's going to do you a damn bit of good dwelling on how rotten you are when you're still a slightly soiled lamb ripe for slaughter if you're not careful." He sighed and shifted, stretching out his long legs toward her, and surely the way his robe fell away from the scar tissue of his Dark Mark when he slung his arm across the back of the couch could not have been accidental. "What do you really want me to say? Do you want me to say you'll never have to kill again? I won't - Trelawney might if you ask her nicely."

f. I shall, yes. :)