Timing of the request

[Athena Appleton]

Okay, I was wondering, do you think Dumbledore had already told Snape to kill him before Snape took the unbreakable vow with Narcissa, or after?

I kind of wonder if Snape didn't get caught kind of like a deer in the headlights when she asks for that, realizes what it would look like if he said no, and did it because was cornered, then came to Dumbledore to tell him what happened.

[Broccoli]

I don't think that he came to Dumbledore and told him about the vow before Dumbledore's request, because when Dumbledore made the request it seemed to be a surprise to Snape. I also think he could have found his way out of making the vow if he had wanted to.