Snape's out for himself? Is that all you got?
The dude can teach others to brew glory and fame and stopper death. He's never once, in any of the books, talked about wanting those things for himself. In fact, our only information about what Snape wants is brought to us via other characters who are somewhat unreliable. Snape himself has never given up anything in the way of information. If all he wanted was fame and glory, then he'd have brewed a truckload of all of and moved to Hollywood. Again, what would he be preserving himself for? To live another day so he can annoy Harry Potter. To become the Defense Against the Dark Arts teacher?
And Snape wouldn't be the first character who acted in a manner that goes against the house he was sorted into. the truly horrible Peter Pettigrew was a Gryffindor, lest we not forget.
Snape isn't a classic villain in the way Voldemort is. Classic villains always have a goal in mind. They want something. Voldemort wants eternal life and power and Harry is an impediment to that. One would think that Snape wants Harry dead, but he's spent the last six books saving him from that fate -- which is good for Dumbledore, good for Voldemort, but what does it do for Snape. We already have a character in the book who's out for himself ... Peter Pettigrew, who Snape also hates. What would be the point of a character with the same goal?