The High Sheriff of London: he's probably a respectable guy, right? If you've learned anything by now from The Threepenny Opera, it's that no one is respectable, and least of all a cop. As constable, Jackie Brown takes bribes from guys like Mac to look the other way while they commit their crimes. He doesn't worry a bit about the morality of the situation.
Jackie and Mac go way back. They "served in India together, soldiers both of us" (1.2.411-412), as Mac reminds him at his wedding. And that war-buddy friendship is the basis for the unfairness, special treatment, corruption, and downright incompetence that characterizes their cops-and-robbers routine in the play.
Brown promises Mac, "There's nothing whatsoever on record against you at Scotland Yard" (1.2.480-481) because he's "taken care of that" (1.2.483). And it's a good thing, for Mac if not the general public, because Mac's record is a mile long. So is Brown's friendship with Mac really all it takes to erase such crimes?
Nope. It's all about the money. Tiger Brown is a sold-out sell-out. Right before his old buddy Mac is to be executed, he goes through a long accounting exercise where the two make sure that all the bribes Mac owes Brown are paid for before the trip to the gallows:
MAC. The accounts, Brown.
BROWN. Very well, if you insist. Well, first of all the rewards for murderers arrested thanks to you or your men. The Treasury paid you a total of…
MAC. Three instances at forty pounds a piece, that makes a hundred and twenty pounds. One quarter for you comes to thirty pounds, so that's what we owe you. (3.9.183-189)
Mac "insists" because he's trying to make his friend feel guilty for letting him hang, but even so, the brazenness is impressive.
A play about London's most wanted criminal and its high sheriff might put you in the mood for some chase scenes or cops-and-robbers action, but Jackie Brown is so corrupt that the only conflict he has with Mac are these passive-aggressive scenes.