The context is that the Lawful-Neutral leader of the city-state has been framed and thrown in the dungeons by a shadowy conspiracy of powerful figures. The printing press was just recently invented the first character accidentally inventing Journalism wants to break the story while a coworker is arguing that it's not relevant to the citizens.
Expanded portion:
> "Someone has to care about the... the big truth. What Vetinari mostly does not do is a lot of harm. We’ve had rulers who were completely crazy and very, very nasty. And it wasn’t that long ago, either. Vetinari might not be ‘a very nice man,’ but I had breakfast today with someone who'd be a lot worse if he ran the city, and there are lots more like him. And what’s happening now is wrong. And as for your damn parrot fanciers [...]"
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With respect to "we've had rulers", a bit from a previous book Men At Arms:
> "[...] He wielded the axe, you know. No-one else'd do it. It was a king's neck, after all. Kings are," he spat the word, "special. Even after they'd seen the... private rooms, and cleaned up the... bits. Even then. No-one'd clean up the world. But he took the axe and cursed them all and did it."
> "What king was it?" said Carrot.
> "Lorenzo the Kind," said Vimes, distantly.
> "I've seen his picture in the palace museum," said Carrot. "A fat old man. Surrounded by lots of children."
> "Oh yes," said Vimes, carefully. "He was very fond of children."