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 PratchettI know that I am a small, weak man, but I have amassed a large library; I dream of dangerous places.
Terry PratchettCurrently, it was leading him through a neighborhood that was on the downside of whatever curve you hoped you'd bought your property on the upside of. Graffiti and garbage were everywhere here. They were everywhere in the city, if it came to that, but elsewhere the garbage was better quality, and the graffiti was close to being correctly spelled. The whole area was waiting for something to happen, like a really bad fire.
Terry Pratchett