He didn't administer a reign of terror, just the occasional light shower.

...one day when I was a young boy on holiday in Uberwald I was walking along the bank of a stream when I saw a mother otter with her cubs. A very endearing sight, I'm sure you'll agree, and even as I watched, the mother otter dived into the water and came up with a plump salmon, which she subdued and dragged onto a half submerged log. As she ate it, while of course it was still alive, the body split and I remember to this day the sweet pinkness of its roes as they spilled out, much to the delight of the baby otters, who scrambled over themselves to feed on the delicacy. One of nature's wonders, gentlemen. Mother and children dining upon mother and children. And that is when I first learned about evil. It is built into the very nature of the universe. Every world spins in pain. If there is any kind of supreme being, I told myself, it is up to all of us to become his moral superior.

There's a limit to the power of a spring, no matter how tightly one winds it.'
'Oh, yes. Yes. And you hope that if you wind a spring one way, all its energies will unwind the other way. And sometimes you have to wind the spring as tight as it will go,' said Vetinari, 'and pray it doesn't break.'
His expression changed.

'Oh dear,' he said. 'He didn't thump the wall. I may have gone too far.

The Patrician took a sip of his beer. “I have told this to few people, gentlemen, and I suspect I never will again, but one day when I was a young boy on holiday in Uberwald I was walking along the bank of a stream when I saw a mother otter with her cubs. A very endearing sight, I’m sure you will agree, and even as I watched, the mother otter dived into the water and came up with a plump salmon, which she subdued and dragged on to a half-submerged log. As she ate it, while of course it was still alive, the body split and I remember to this day the sweet pinkness of its roes as they spilled out, much to the delight of the baby otters, who scrambled over themselves to feed on the delicacy. One of nature’s wonders, gentlemen: mother and children dining on mother and children. And that’s when I first learned about evil. It is built into the nature of the universe. Every world spins in pain. If there is any kind of supreme being, I told myself, it is up to all of us to become his moral superior.

We can't just drop everything, sir!"
"Mister Lipwig. Is there something in the word 'tyrant' you do not understand?

I believe you find life such a problem because you think there are good people and bad people. You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides.