Another HBP Theory
Perhaps Snape has given an Unbreakable Vow to Dumbledore. Perhaps the vow is something where, while he may have to do unsightly things as a double agent, ultimately his motivations will be to defeat Voldemort.
I think that his Unbreakable Vow to Narcissa could fall under that, without breaking such a Vow to Dumbledore.
I think that his Unbreakable Vow to Narcissa could fall under that, without breaking such a Vow to Dumbledore.