Write about either the most sinister character in the most innocent setting or the most innocent character in the most sinister setting. Perhaps a serial killer lives in suburbia across the street from a playground. Maybe a bright-eyed, job-seeking college graduate finds themselves in the red light district. Whatever the mix, make it clear why the sinister elements are sinister and why the innocent elements are innocent. Discover how the two opposites intermingle and co-exist, as well as how they effect each other as you write your story. Give this exercise at least fifteen minutes of your time.
Write it up and see what happens.