Phantom in the night - Sherrilyn Kenyon

Phantom In The Night features a female agent from the Bureau of American Defense, who's after a home-grown terrorist...

Nathan Drake is a typical overachiever, graduating high school at seventeen and joining the Army to help support his ailing mother and twin brother. When his brother Jamie is arrested for drug charges, Nathan knows the stress would kill their mother so he makes a deal with his brother. He'll serve the eighteen-month sentence provided Jamie promises to clean up his act.

But all doesn't go as planned. Nathan ends up in a fight that results with his having to serve the entire six-year sentence. A year later, his mother succumbs to her cancer and dies. Three days before his release, Jamie is killed by someone from his drug-dealing past. His life in shambles, his family gone, Nathan has no reason to live. Donning the guise of a "ghost," he systematically begins assassinating every criminal who had a part in his brother's death...