Retribution by Sherrilyn Kenyon

Harm no human...

A hired gunslinger, William Jessup Brady lived his life with one foot in the grave. He believed that every life had a price. Until the day when he finally found a reason to live. In one single act of brutal betrayal, he lost everything, including his life. Brought back by a Greek goddess to be one of her Dark-Hunters, he gave his immortal soul for vengeance and swore he'd spend eternity protecting the humans he'd once considered prey.

Orphaned as a toddler, Abigail Yager was taken in by a family of vampires and raised on one belief - Dark-Hunters are the evil who prey on both their people and mankind, and they must all be destroyed. While protecting her adoptive race, she has spent her life eliminating the Dark-Hunters and training for the day when she meeting the man who killed her family: Jess Brady.

A gun in the hand is worth two in the holster....

Jess has been charged with finding and terminating the creature who's assassinating Dark-Hunters. The last thing he expects to find is a human face behind the killings, but when that face bears a striking resemblance to the one who murdered him centuries ago, he knows something evil is going on. He also knows he's not the one who killed her parents. But Abigail refuses to believe the truth and is determined to see him dead once and for all.

Brought together by an angry god and chased by ancient enemies out to kill them both, they must find a way to overcome their mutual hatred or watch as one of the darkest of powers rises and kills both the races they've sworn to protect.

This book is part of the "Dark-Hunter" series.
Here are some other books from this series: