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 PratchettRemember - that which does not kill us can only make us stronger. And that which does kill us leaves us dead!
Terry PratchettOf course, Ankh-Morpork's citizens had always claimed that the river water was incredibly pure. Any water that had passed through so many kidneys, they reasoned, had to be very pure indeed.
Terry Pratchett