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 PratchettSomewhere around the place I've got an unfinished short story about Schrodinger's Dog; it was mostly moaning about all the attention the cat was getting.
Terry PratchettBlessings be on this house," Granny said, perfunctorily. It was always a good opening remark for a witch. It concentrated people's minds on what other things might be on this house.
Terry Pratchett