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. '
Terrible things always seemed to happen to hitchhikers in movies - including my own. It has always been glamorous and dangerous and scary and sexy.
Don't pick up hitchhikers!"- D. Adams