It’s a noir staple to open with a bit of narration, but once the nameless hit-man protagonist of David Fincher’s “The Killer” starts gabbing, he doesn’t stop. As Fincher’s assassin (Michael Fassbender) awaits his target from a high, unfinished floor in a Paris building that looks out on the home of his mark, his inner monologue runs with a smooth, affectless monotone. His musings are a mix of professional tips (“Anticipate, don’t improvise”), nihilistic existential observations (“Most people refuse to believe that the great beyond is anything more than a cold, infinite void”) and sincere self-reflections (“I’m not exceptional, I’m just apart”).