But Snape did get an entire book named after him. After being a horrid teacher to Harry, he becomes the teacher Harry looks up to most, via the book. As far as I'm concerned, the ambiguity is removed when Harry learns who is in fact the half-blood prince: Snape has to be good since he's taught him so much, and all appearances of the contrary are misleading on purpose. We learn that purpose in DH - Snape doesn't want to feel any sympathy for a living Horcrux and thus has to force himself to be mean, all the more so since that Horcrux has Lily's eyes. Simple motivation => complex attitude.