Production Design
It's a Very Harry World, And We're All Just Living in It
The film is pretty tightly hitched up to Harry's perspective. What does that mean? Well, basically that we don't really get to know too much more than Harry does. Aside from a conversation among Narcissa Malfoy, Snape, and Bellatrix Lestrange at the beginning of the film, we're mostly following Harry and his crew, and only finding out the stuff that he finds out.
So, since he doesn't know what's really up with Snape, we don't get to know either. Is he loyal to Dumbledore? Faking it? Like Harry, we can't really tell, especially since we mostly hear or see Snape only when Harry's around. Looks like we'll have to wait until Harry knows the truth to get the full story. We really hope that's soon.