This year at Conclave I'll be making a variety of presentations including "Werewolves of Michigan," "Michigan's Most Haunted," "Secrets of the Emerald Tablet" and "Using Female Assassins & Secret Societies in Your Novel." Of these topics, I think Female Assassins will be the most well attended.
The last ten years have seen an explosion of female assassins in both print and movies. The movies have been particularly generous in this regard- "Kill Bill," "Mr. & Mrs. Smith," "Aeon Flux" and "Ultraviolet" come to mind immediately. There's a reason for this, and it making use of this trend can really ratchet up the tension in your novel. And if you use it right, there's a big chance you will have better luck selling your story.
Think of it this way- a lot money is spent by the entertainment industry on stories involving female assassins and secret societies. So why haven't you included one or the other in your own fiction?