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 PratchettIf you have let pride get the better of you, then you have already lost, but if you grab pride by the scruff of the neck and ride it like a stallion, then you may have already won.
Terry Pratchett… people didn't seem to be able to remember what it was like with the elves around. Life was certainly more interesting then, but usually because it was shorter. And it was more colorful, if you liked the color of blood.
Terry Pratchett