A Cold Dark Place by Toni Anderson

She’s an FBI agent hunting her sister’s killer.
He’s an assassin who’ll die to keep her safe.
His secret will destroy them both.

Former CIA assassin Alex Parker works for a clandestine government organization hell-bent on taking out serial killers and pedophiles before they enter the justice system. Alex doesn’t enjoy killing, but he’s damn good at it.
FBI Special Agent Mallory Rooney has spent years hunting the lowlife who abducted her identical twin sister eighteen years ago. Now, during an on-going serial killer investigation, Mallory begins to suspect there’s a vigilante operating outside the law.
When Mallory starts asking questions, Alex is ordered to watch her. As soon as they meet, the two begin to fall in love. But the lies and betrayals that define Alex’s life threaten to destroy them both—especially when the man who stole her sister all those years ago makes Mallory his next target.