Wow! Sweet, hot and somewhat dark. What an eerie ending! I'd find it truly dark except - somehow, I think Harry means no harm. He's just found, or fallen into, a strange way of bringing people close to him. It's creepy that so many of them are bad - but then there are the good ones, too.
And at least this time, Severus doesn't have to feel like he was wrong and bad and evil to get sucked into Dark Magic. The whole air of innocence you cast across this, with Harry seeming not to know what he's doing, and Snape sincerely trying to resist it, make it a more complicated mix of good/bad, Light/Dark. Looking at some of the other comments, I see we're having to *think* about this fic, and we've come up with a lot of different responses and interpretations! I hope that pleases you, as the author.
I very much enjoyed the way you wrote this. Very spare prose in places, but with those wonderful touches of color, of emphasis and inner thoughts. The metaphor of Harry as vine, as Devil's Snare, is wonderful. I like how so many sentences express the same ambivalence as Snape is going through, the "is it? or not? or *what*?", the contradictions between people's surfaces and what's inside, their actions and their dreams. Like here: It goes as planned, except it doesn't. Severus casts the Legilimancy spell wordlessly, though he must brandish his wand. ... Potter smiles and nods, but the contours of his mind are not contours at all, they are smoothed over...
Just in that one paragraph, four sentences, there are three twists, three surprises, three things that look like one thing but turn out to be another. Or here, again: "Severus pushes Potter away, but only so he can herd him toward the bed."
Nothing is as straightforward as it looked, and I think by writing it in an uncomplicated way, you made this story of shifts and surprises both clear, and also more vividly shocking as it just keeps slithering out of Snape's control, into ever stranger territory.