Goodness, you're having a strong reaction to the character.

Instead of getting his shit together and making a life for himself

This really isn't that possible for him. He's bound by his guilt about Lily's death and his service to Dumbledore to protect Harry. He is much more dickish and petty in the books than in the movies, but his entire story does a pretty good job of showing that he's a rather tragic character.

He's ambitious and talented and because he made the mistake of becoming a Deatheater - a decision he regretted and which contributed to Lily's death - he has squealched the rest of his life. When Voldemort returns Dumbledore enlists Snape to work as a double agent, using him (as Dumbledore uses many people) by exploiting his sense of guilt.

He honors his promise to Dumbledore to protect Harry even though he resents Harry deeply.

Goodness, you're having a strong reaction to the character.

Well, I'm not emotionally involved, really: but the dude's a teacher. He's not supposed to be screaming at his students, or taking out his own childhood traumas on them.

And I'm trying to figure out why he's so popular, when at this point (three books into the series), Snape has been consistently a dangerous threat to the protagonists, and if not evil, at least completely misled. Because at this point, we don't know any of his backstory other than that he hated James & the others. So why the fannish love?

Wait, have you not read the last book? Because that's where he was truly redeemed (in my eyes at least.) In ways i can't explain without total spoilery

No, I've read them all, but I'm rereading from the beginning. And I only ever read them each once, as they came out, so most of the details are pretty fuzzy.

Wait, have you not read the last book?

That is a key question, because it does address Snape's backstory and motives in detail.

And I'm trying to figure out why he's so popular, when at this point (three books into the series), Snape has been consistently a dangerous threat to the protagonists, and if not evil, at least completely misled. Because at this point, we don't know any of his backstory other than that he hated James & the others. So why the fannish love?

Because he's flawed and damaged and people like the broken teacup characters?

people like the broken teacup characters?

Too true, but there's quite a gulf between Snape and Rodney McKay, or even Snape and Alex Krycek. (Nick Lea, for one!)

And I only ever read them each once, as they came out, so most of the details are pretty fuzzy.

Well, when Harry receives Snape's memories in the last book you do understand how tortured Snape's existence has been. That he's both arrogant and filled with self loathing. That he's been very honorable (though mean). That he's been true to the one thing that mattered to him (Lilly). That he's very bravely endured incredible trials because of his promise to Dumbledore.

And while I think that Snape's actions in the earlier books don't necessarily indicate all this, JKR does drop hints. Particularly when Harry is studying occulemency and receives the memory of Snape's humiliation by his father.

But again, in Snape's mind, that all led back to Lily and Snape's idea of his worst day being his "MUDBLOOD" hollering at Lily and ruingin any small remains of their friendship, prior to his buddying up with the Young Republicans Deatheaters, not necessarily the embarassment caused by James.

People are also convinced that Draco is a sympathetic character.

I don't see how Snape's morality trumps Krycek's, though.

Oh, I totally feel bad for Draco in book 6, especially. Is that different than being a sympathetic character?