Pratchett of the (Insert Random Measure of Time)

And he was not certain, not certain at all, what he'd do if the prisoner gave him any lip or tried to be smart. Beating people up in little rooms... he knew where that led. And if you did it for a good reason, you'd do it for a bad one. You couldn't say "we're the good guys" and do bad-guy things.

- Thud, p 256, after the bit with the dwarfs in the basement, when the mine was cracked open and the bodies were found, but before they went off to... oh just read the whole thing. Its worth it.