Well seeing Era has made a post on the subject and that this post has been sitting in my drafts for a week now I might as well post it with the book due Saturday.
I can't be bothered making as many predictions as Era but based on my rereading of the Half-blood prince last week I suspect we may find out something along these lines.
Snape is Good, and following DD's orders. Various things in Half-blood suggest this.
He makes the unbreakable oath in HB, to help Mal, knowing that DD would expect him too or possibly as he's already discussed this with DD.
The reason being that DD suspects he's going to be killed soon, and that Mal has been ordered to do it and would prefer Snape to do it rather than force Mal to do something truly evil.
Any way I'm looking forward to Saturday!