The first stanza of the first song of the play's prologue compares the protagonist, Mac, to the shark. Both of them carry sharp weapons (the shark, teeth; Mac, a knife), but with one important difference. The shark displays his teeth for everyone to see, but Mac keeps his knife hidden.
PEACHUM. We let you off with a thrashing because we had reason to believe you didn't know what's what. But if you show your face again it'll be the chop for you. Got it? (1.1.70-72)
Mr. Peachum is threatening a newbie beggar, who didn't realize that he had to get permission to beg on the streets of London. Peachum explains the punishment, "a thrashing," as though he were going easy on the kid. The next step "the chop," sounds pretty scary, and the whole exchange is just dripping with casual violence.
MAC. My orders were: avoid bloodshed. It makes me sick to think of it. You'll never make business men! Cannibals, perhaps, but not business men! (1.2.49-51)
Mac ordered his men to go steal a bunch of furniture and supplies without hurting anyone. We know that his feeling "sick" is ironic, because we know that he himself sheds blood constantly (not his own). He compares the men to cannibals, who eat other human beings, saying they could never be businessmen, but that irony makes us wonder whether the play isn't actually calling businessmen cannibals.
And they'll ask: what kind of a bang was that?
And they'll see me as I stand beside the window
And they'll say: what has she got to smile at? (1.2.329-331)
In the "Pirate Jenny" song, Polly sings about being a barmaid who gets her revenge, big time, when a ship comes to town and blows the whole place to smithereens. The repetition of "And they'll…" makes the verse sing-songy and childlike, but the subject matter is actually quite violent.
POLLY. Perhaps there wasn't yesterday, but suddenly today there's an awful lot. You—I've brought the charges with me, I don't even know if I can get them straight, the list goes on so. You've killed two shopkeepers, more than thirty burglaries, twenty-three hold-ups, and God knows how many acts of arson, attempted murder, forgery and perjury, all within eighteen months. You're a dreadful man. (2.4.18-24)
Because Mac's such good friends with the chief of police, up until yesterday his violent acts had gone unpunished. But now they're all on the books, suddenly, and Polly's reading of the list brings to light just how bad old Mackie really is. The numbers seem absurd. That official record, compared to her moral judgment ("you're a dreadful man"), is hard to reconcile.
There goes a man who's won his spurs in battle
The butcher, he. And all the others, cattle. (2.4.187-188)
Who else could be the subject of this song but Mac? He's compared to a warrior who wins "spurs in battle," as though life were a war and Mac were winning it. The really creepy metaphor, though, compares Mac to a butcher and everyone else to cattle, which means that he has utter control over the lives of others and has no problem bringing those lives to an end.
JENNY. I'd ask him straight to say what he thought he was
Then he'd lash out and knock me headlong down the stairs.
I had the bruises off and on for years. (2.5.105-107)
Besides the random, criminal violence we've been hearing about from Mac, he also commits violent act in his personal relationships. When he was living with Jenny he abused her horribly. The bruises were a permanent mark of his violence, almost like a brand marking her as his property.
JENNY. Once I was pregnant, so the doctor said.
MAC. So we reversed positions on the bed.
JENNY. He thought his weight would make it premature.
MAC. But in the end we flushed it down the sewer. (2.5.120-127)
When Jenny becomes pregnant, Mac's solution is to cause an abortion by squashing Jenny and the fetus. When that aggressive act doesn't work, they commit another violence against the newborn by flushing it down the sewer, killing their child. Mac's violence seems to know no bounds.
POLLY. Just a second. I only have to…I only have to tell him something…Really…it's very important.
MRS. PEACHUM. giving her a box on the ear. Well, this is important too. Get going!
POLLY. Oh, Mac! She is dragged away. (2.6.245-246)
Mac and Jenny aren't the only one with domestic violence issues. Mrs. Peachum, too, has no problem beating her grown daughter, Polly. "A box on the ear" is a slap, and the way Mrs. Peachum uses it to control Polly shows how commonplace violence is among the play's characters.
PEACHUM. But as we want to keep our fingers clean
And you're the people we can't risk offending
We thought we'd better do without this scene
And substitute instead a different ending. (3.9.333-336)
Though the play has been rife with violence, suddenly Peachum declares that they'll spare the audience the violence of a hanging and change the ending. Of course this is ironic, meant to make the audience members think about what they've seen and whether it's something that's natural in humanity or possible to change.