Firstly, what we were given in the book was mere basic background, but it provided enough for us to get an initial idea of what to expect. Bellatrix was born into a family obsessed with blood purity and punished those deemed ‘traitors’ for rebelling against these ideas. We know this from family accounts, and the Black family crest which reads ‘always pure’ in French.

Now. She married her husband (a pure blood wizard) because it was expected. She never speaks of him, shows no concern he’s in prison and really, ultimately, only needed him to get in with the Dark Lord.

The woman herself - she is a sadist. She enjoys hurting others and gets a great amount of satisfaction (probably both emotionally, and sexually) from inflicting pain/torture on others. I don’t, however, think she’s a psychopath. Bella feels emotion. She’s capable of love, of concern and knows the difference between right and wrong. She just doesn’t care – mind marred by her blood ideals.

I think she's always been like this, since childhood. She's the type of kid you can envisage playing with bugs until they day - just for the hell of it. I don't think that enjoyment of power over another being ever left her.
Contrastingly, Voldemort is not capable of feeling emotion. Why, then, does he chose to rescue her from the Ministry rather then letting her go back to Azkaban (will come on to the effects of prison on her in a second)? I think, although he can’t feel love, he feels something when he’s with her. And that slight elevation in emotion (even though it’s not identifiable as love to him, or anything positive really) it’s enough for him to want to keep her around.

I think she needs something like this. Someone to *want* her... I"m not sure if Rodolphus ever did. And I'm not sure where that stemmed from. I mean, she had a privileged childhood, but it's possible the love and care was replaced with riches and possesions.

So anyways, Bellatrix picked up on that, and her previous devotion to his cause then became a devotion to him. She loved him. Fanatically (she doesn’t seem to do things by halves - childhood link to perfection/need for power and control).

Azkaban reportedly took a beautiful woman, and turned her gaunt, pale and manic. Emotionally, she was probably pushed over the edge. Isolation and torture will have set up her need for contact with other people, leading to a heightened sexual/seductive nature to her, one that she’s probably not aware of because she’s lost the ability to know where the social line is.

The fact that Voldemort will have praised her for her loyalty to him will only have cemented her ideas that what she does is the right thing. On some level I think she knows she can never have the man she wants. So she compensates, flirting, working, torturing for the attention off him that he doesn’t give.

As for her past with this, I agree to some extent. I've always seen her as very close with Narcissa, and probably Andromeda before she was shunned (part of me thinks that was hard for Bella to do). I think she juts got caught up in it all - when you've had family values printed on you from an early age it's near impossible to ignore them. Rodolphus and Voldemort furthered that and further fragmented her already precariously held together personality.