Interesting stuff. I too, find Bella both irritating and intriguing, and look forward to seeing how the character's story develops.

"Family" covers a lot of possibilities but since this is Supernatural, it's probably safe to assume that she killed either a sibling, a parent or her child. The latter is unlikely because the character is relatively young.

Well, she appears to be in her mid-twenties at least, which is plenty old enough to have a child -- though it would have to have been pretty young when she killed it/caused its death.

And the way that scene played out, Dean asks Bella something like "Who did you kill? Parent? Sibling?" He conspicuously never mentions the possibility of a child, which immediately made me think that must be the answer.

I agree with you that she seems to protest too much with her "hunters are all revenge-driven sociopaths" spiel. There's gotta be some personal history there.

It makes me wonder if, in the end, it will be Bella who holds the key to unlocking Dean's contract.

Oooh, I haven't considered that! I think that would be incredibly cool.