If there are unwritten rules for contract killers, one might be, always carry a gun on a job. Brad Pitt, codename Ladybug, breaks this rule in the very first minutes of “Bullet Train,” a new film where he stars as a contract killer who isn’t really a big fan of violence. Clad in a fisherman’s hat and a trench coat, he creeps through a big Japanese metropolis, wanting to change his life. “I’m ready. You’ll get a new and improved version of me,” he says in a call with his contact. He feels bad that so far, all his assignments have gone off the rails. Ladybug’s trying out a new mantra: “If you br…