I think I might prefer if Bela offers up a solution as a way to keep Dean from killing her, especially rather than out of a romantic or even purely sexual relationship.

I'm not sure I like what that scenario would say about Dean. I mean, I think he meant it when he said he'd kill her, but that's because she, in effect, tried to kill Sam. I can't see him committing murder for anything less than that.

But I never meant to suggest a romantic relationship between them. I just meant she needs to care whether he lives or dies. "Care" can mean a lot of different things.

I think Ruby's going to dangle the bait just long enough to push Sam significantly into the dark side

I think that's her intention. I haven't figured out why, yet.

Of course, if I were writing it, there would be no solution. Dean would go to hell, and the next season would be about getting him back. I doubt that's the way it'll go down - it makes dramatic sense but in terms of TV production it would mean a bunch of episodes with no Dean. They won't do that.