Symbolism, Imagery, Allegory
"See the shark with teeth like razors" (P.5)—the play's first image is one of its most famous, working its way into the heads of fans of Sinatra, Bobby Darin, Louis Armstrong, and tons of other singers. It compares Mac the Knife, the infamous killer, to a dangerous shark.
What's important, though, isn't necessarily what Mac and the shark have in common; it's what they don't. The idea is that the shark shows his teeth for everyone to see. You can expect danger when you see a shark.
Mac, on the other hand, "has got a knife, but/ Not in such an obvious place" (P.7-8). Mac's danger isn't in his weapons; it's that he hides his violence behind a seemingly friendly demeanor. He "wears white kid gloves which/ Give the minimum away" (P.11-12). The really awful thing about Mac, what the play criticizes, is hypocrisy.