Zsadist, such a misnomer, more like a masochist (needs the pain, but not in a sexual way), through no fault of his own. As a child Zsadist had been kidnapped and eventually sold as a blood slave. However, once he went through the transition, the mistress of the house found much more interesting things to do to her new plaything. After a century of being used he was finally rescued by his twin brother, but the years of abuse have left him scarred physically, but more importantly his soul. In the previous book he was unable to bear being touched he was unable handle the emotions when Bella showed that she wanted him. Then when she was kidnapped he really went crazy. In the beginning of this book we find him still looking for her. 
 We also start to get a glimmer of the nonwarrior vampires. Males are overprotective of the females in their care and some are willing to put them into vampire Purdah to keep them and the family name clean. As I am in the mist of rereading the series, I know where this begins to lead, which is one of the great things about the series. Many of the major arcs in the other books start as little arcs in previous books and often have ripples in later books. 
 As for Zsadist, as with so many others who's trust and bodies have been violated repeatedly, he can't get past the thought that his body can never get clean, that he because of the abuse that he will always be unworthy of love. What I liked about this is that he didn't just get "cured" of this by his bonding with Bella, but that they will still have issues because of it, but together they will work through it. 
 Which brings me to another reason why I like this series. While the warriors seem to bond almost immediately it is still not a destined thing. Even after having bonded with Bella Zsadist kept trying to send her away so she could have a male of worth. The fact that it is the choices of the couples makes this so much more than just a "you're mine, we will be together forever thing". Even after bonding the pair have to work at their relationship.