Event Horizon. Singularity, #5

They're trapped in the middle of an alien war-and their only hope of salvation is from the most unlikely of sources. With the survivors of the diplomatic ship finally safe, head diplomat and former Chief Justice Alba Espina can at last turn her attention to finding them a way home. Or so she thinks. And then the vicious raiders who hunt the edges of the alien system turn their sights on the alien capital, and once again, human lives are a potential bargaining chip for the aliens' survival.
Now, as Alba struggles to convince the aliens that humanity is worth saving, charming assassin Savina Maya and the government agent sent to hunt her down are thrust into a desperate, brutal guerrilla war to protect the human settlements and take down the aliens and raiders arrayed against them. In the meantime, field-scientist Aran Romeu has finally found the cure he's been desperately searching for-but doing so required him to sign on as raider crew.
Now he's got to find a way to keep his exceptionally bloodthirsty raider captain from burning the entire system to the ground, before everyone he cares about is killed.