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 Pratchett[Science fiction is] out in the mainstream now. You can tell by the way mainstream literary authors pillage SF while denying they're writing it!
Terry PratchettAnd then you bit onto them, and learned once again that Cut-me-own-Throat Dibbler could find a use for bits of an animal that the animal didn't know it had got. Dibbler had worked out that with enough fried onions and mustard people would eat anything.
Terry PratchettYou can't make people happy by law. If you said to a bunch of average people two hundred years ago "Would you be happy in a world where medical care is widely available, houses are clean, the world's music and sights and foods can be brought into your home at small cost, traveling even 100 miles is easy, childbirth is generally not fatal to mother or child, you don't have to die of dental abscesses and you don't have to do what the squire tells you" they'd think you were talking about the New Jerusalem and say "yes."
Terry Pratchett