I despised Severus a bit at how easily he was taken in just by luxury, and then the stupidity of the Unbreakable Vow almost made me weep. It works well in your story, though, as Severus will have great cause to regret this the next day. Or perhaps he sees the events of Boxing Day through a different lens, now that he knows he's leaving? I do prefer to imagine that Snape wasn't 'tricked' into making his choice to join the Dark Lord, but your Severus, so thirsty for material things, overwhelmed by his senses, dulled by his toying with the dark arts, well, it's just about what he'd deserve.
Really loved the juxtaposition of the two families, and how one was really no happier than the other. Very well-written, lovely detail, mystery author!