I think when people mean that Discworld books have become darker they really mean the series is growing up. In 'The Colour of Magic' most of the city is set alight. It's a joke, in much the same way that the Earth is destroyed almost at the start of Douglas Adams's 'The Hitchhikers Guide to the Galaxy.'
Terry PratchettSybil's female forebears had valiantly backed up their husbands as distant embassies were besieged, had given birth on a camel or in the shade of a stricken elephant, had handed around the little gold chocolates while trolls were trying to break into the compound, or had merely stayed at home and nursed such bits of husbands and sons as made it back from endless little wars. The result was a species of woman who, when duty called, turned into solid steel.
Terry PratchettHistorical Re-creation, he thought glumly, as they picked their way across, under, over or through the boulders and insect-buzzing heaps of splintered timber, with streamlets running everywhere. Only we do it with people dressing up and running around with blunt weapons, and people selling hot dogs, and the girls all miserable because they can only dress up as wenches, wenching being the only job available to women in the olden days.
Terry PratchettSome police forces would believe anything. Not the Metropolitan police, though. The Met was the hardest, most cynically pragmatic, most stubbornly down-to-earth police force in Britain. It would take a lot to faze a copper from the Met. It would take, for example, a huge, battered car that was nothing more nor less than a fireball, a blazing, roaring, twisted metal lemon from Hell, driven by a grinning lunatic in sunglasses, sitting amid the flames, trailing thick black smoke, coming straight at them through the lashing rain and wind at eighty miles an hour.That would do it every time.
Terry Pratchett