HBP: Chapter 2 - Spinner's End

Another chapter, not from the vantage point of Harry and hosts David and Kyle, are impressed. In this chapter, they discuss a meeting between Snape, Narcissa Malfoy and Bellatrix Lestrange at Spinner's End. It starts with intrigue and ends with some pretty lofty promises. What's going on, and how does this chapter do such a good job of shaping Snape's character for the rest of the series?