Season of the Machete by James Patterson

Cool and glamorous, they appear to be a successful couple on a holiday. Yet Damian and Carrie Rose are psychopathic murderers for hire. On this picture-perfect vacation island, their target is Peter Macdonald, a dashing young American who forsakes a life of leisure to confront cold-blooded terror. But when they clash in a shocking endgame, a hideous truth will emerge - one that can destroy them all.