Its been awhile since i read the book Bitten but from what i remember the theme is definitely Elena being caught between her civilised human self and her beastial nature and what happens when you dont balance the two which is a classic theme in many werewolf novels as well as Jekyl and Hyde though its usually male werewolfs who get to struggle with this.
Basically Elena is a werwolf but she hasnt really embraced being a werwolf and is trying to live a perfectly civilised human life in teh city whilst ignoring and suppressing her beastial nature as much as she can. Going back to the pack to help them out also means being in the countryside and the company of other wolves which call out the wolf side of her that shes been suppressing up til now. And no the author doesnt go down the cliche route of having her beast side reassert itself in a dramatic and tragic way. Its a lot more subtle and complex a situation than that.
I dont want to go into too much detail or reveal what the threat to the pack is (which fits perfectly with the theme of civilised vs beastial self) as i assume people who havent read teh books, are interested in watching the series, and dotn want to eb spoiled are gonna read this thread. So im gonna shut up now.