The reasons individuals become involved with the Twilight's Hammer are numerous. Archbishop Benedictus turned his back on the Light and become the Twilight Father out of despair after seeing Deathwing's face. Others just like the idea of anarchy and chaos. While still more are drawn to the idea of power or death.
Asira Sunbright would have been your average blood elf blade-for-hire, winning some small acclaim in the gladiatorial ring if Cho'gall had not taken notice of her. At first when approached for some small jobs she was hesitant. The Twilight's Hammer didn't have the best reputation as employers. Not that they weren't generous, or their tasks challenging. It was just that they their handlers tended to be rather unstable. Eventually greed over came her miss givings.
Eventually Asira's close and continued association to the corrosive influence of the Twilight's Hammer began to impact her personally. The jobs became less about the money and more about the thrill, the challenge, the death itself. The transformation from hired hand to fully committed member came when she chose to change her name to "Dawnslayer". A major step in elf society, signalling something essence altering had occurred within an individual's life, making their previous exsistence "dead".
By this time she had become an assassin of note within the Cult, frequently gaining both the highest fees as well as the most sensitive assignments. When we first encounter her or even learn of her existence it is because she has been given the task of eliminating Thrall and retrieving the Dragon Soul as he makes his way across Dragonblight during his journey to Wrymrest Temple. Time is rapidly running out for the Twilight's Hammer and it is interesting that only now do they think to send a rogue in to do what rogues are best at, take out a single target and snatch up an item. It only goes to show how mad these guys are.
Sadly, Asira chooses to confront Thrall and his escort out in the open and from the front, never a rogue's strongest position. In addition she is out numbered six to one. Perhaps all of her time spent in the Twilight's Hammers company has instilled a false sense of superiority, or she has just gotten cocky on her past success. About they only ones she manages to irritate are the casters as she puts her tools to use tossing her version of smoke bomb and has still been able to maintain the ability to silence with a thrown knife. Yet she never uses the major tool of rogues....stealth. As a result her career ends at the hands of Thrall and his companions far from her birthplace at Silvermoon.