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.'
And therefore education at the University mostly worked by the age-old method of putting a lot of young people in the vicinity of a lot of books and hoping that something would pass from one to the other, while the actual young people put themselves in the vicinity of inns and taverns for exactly the same reason.
All I am really promoting in the books is the Golden Rule, which I hope everybody knows to be "do as you would be done by." It has one or 2 flaws, but it is a good soundbite. Evil starts when you treat other people as things. There are perhaps worse crimes, but they begin when you treat other people as things.
People flock in, nevertheless, in search of answers to those questions only librarians are considered to be able to answer, such as "Is this the laundry?" "How do you spell surreptitious?" and, on a regular basis, "Do you have a book I remember reading once? It had a red cover and it turned out they were twins.
Let's say I've directed that [writing] energy into writing my latest book but suddenly, I really want to write about an onion. I don't say to myself, "No, you have stay on the subject," because I know that the longer I stay on the subject the more boring I get. So, if my mind wants to write about an onion, it might be a deeper way to go into what I'm working on, even though it might seem irrelevant. This is how I've learned to follow my mind.
Friends open the door for me to write. Then I get paid attention to and it allows me to write other books. The Great Spring and the thirtieth anniversary of Bones just came out and while I'm happy and excited about that, I've already finished a new book. That's what practice does. You don't get caught.