It's from one of his Discworld books.
Like much his work it's both funny and insightful !
You see, the only thing the good people are good at is overthrowing the bad people. And you're good at that, I'll grant you. But the trouble is it's the only thing you're good at. One day it's the ringing of the bells and the casting down of the evil tyrant, and the next it's everyone sitting around complaining that ever since the tyrant was overthrown no one's been taking out the trash. Because the bad people know how to plan. It's part of the specification, you might say. Every evil tyrant has a plan to rule the world. The good people don't seem to have the knack.”
It is from the novel Guards! Guards! and is said by Lord Vetinari.
