Luna and Neville (and Harry, of course) are both often bullied and publicly humiliated, either by teachers, grandmothers, other relatives or schoolmates, but none of them turns to the dark arts, as far as we know. Neville's childhood wasn't exactly sunshine and roses either. I do not see Hogwarts making Snape into anything. He is what he chose to be.

Well said! I get so tired of people holding up the "prank" as an excuse for Snape to be the way he is.

I also agree that there isn't much to recommend him at the moment. But he did show compassion and caring in his Vow to protect Draco. Misplaced? Yes. He would be better off convincing Draco to stay away from LV altogether, but no one that is completely self-serving would have entered into such a contract. JKR also said that he has been loved before, which makes him worse than LV in a way. But maybe, (and here's me hoping,) that is what will make the difference in the end--that he has been loved and has loved in the past. As you said though, it will come down to his choice.