Sweet Hellion by Tarah Scott

Branded a deserter, Rhys Macleod has been stripped of his command, his title, and has been dishonorably discharged from the army. He has retreated to the quiet solitude of Blackstone Abbey in hopes of forgetting the country and family that has abandoned him. But his peace is shattered when his best friend’s sister shows up to avenge her brother’s death and Rhys comes face to face with his broken promise to care for his friends beautiful sister.
Emma Bamfield refuses to let her brother’s death go unpunished. The deserter who got him and the rest of his unit slaughtered on the battlefield must die. When Emma discovers Captain Rhys Macleod is but an hour’s ride from Inverness, she sets out to shoot him. But shooting a man in the back turns out to be more difficult than Emma expected.