How we cite our quotes: (Act.Scene.Line)
Quote #1
See the sharks with teeth like razors.
All can read his open face.
And Macheath has got a knife, but
Not in such an obvious place. (P.5-8)
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.
Quote #2
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.
Quote #3
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.