Theory about book six, and questions about a certain plot point....
Snape and Unbreakable Vows: Understanding the Past, Implications for the Future
JKR almost never introduces a concept to serve just one purpose. Maybe part of the reason why Dumbledore trusted Snape was because he made some sort of unbreakable vow to be loyal to the Order, or to protect Harry, or something of that sort?
And of course, the whole concept of an Unbreakable Vow raises some questions. Why doesn't Voldemort have all his Deatheaters swear Unbreakable vows to be loyal to him forever? Is that beneath him, or something? As in, it doesn't make him look as good if he can't get his Deatheaters to follow him blindly without an Unbreakable Vow? Why doesn't the Order require all of its members to swear unbreakable vows of some sort? It doesn't seem like it should be considered Dark Magic.... And remember, Fred&George tried to get Ron to swear an Unbreakable Vow when he was five years old. If Unbreakable Vows are known to even kids, what are the chances that they're considered Dark Magic? And again, if the Weasley kids know about Unbreakable Vows, wouldn't it seem that it's a well-known bit of magic that is actually used to some extent? Why isn't it used more often, to prevent traitors? The only thing I can think of that might go some way toward answering these questions is that there might be a danger that an opportunity to fulfill the vow might present itself without the person realizing, and so the person might die without any cause. But I'd think that in order for an the death clause of an unbreakable vow to take effect, the person would have to realize that they were breaking their vow.... Or would they? If a person actually convinced themselves (in error) that it was impossible for them, at that point in time, to fulfill their vow - and so did not fulfill the vow - would they die? What triggers the death, intentions or actuality? There's quite a lot to discuss here about Unbreakable Vows, methinks.