This is by H28 on onedirectionfanfiction.com it's wonderful and I hope you like it as much as me. Recommended for 17+

Working for a world-leading mafia, Harry knows how to kill, how to hunt his victims, and how to avoid any company of any kind. He's used to a lonely, quiet, and harsh environment. He is the best at what he does. And everything he does becomes problematic when he's assigned to protect the boss's daughter from world-known killers. Catalina tries to understand him, eventually failing because instead she falls for a man so willing to resist her. [Contains Mature Content]