At a Stranger’s Mercy by Brittany Cournoyer

Making eye contact with a killer wasn’t exactly how Bradley Danforth had planned to start his day. As an accountant, he craved structure and routine. What he hadn’t anticipated was being part of a real-life murder mystery with him cast in the starring role.
Death and sorrow were part of Max Donohue’s job description, leaving the detective hardened after years on the job. On his latest case, he’s the one stuck protecting the only witness in a sadistic homicide.
One death becomes two… then three. And soon Max and Bradley find themselves playing a deadly game of cat and mouse. Can Max save Bradley from the clutches of a serial killer? Or will the killer finish what he started?