I reckon responsible behavior is something to get when you grow older. Like varicose veins.

[...]And his head is on fire with new things[...]he called himself the little blue hermit, scuttling across the sand in search of a new shell, but now he looks at the sky and knows that no shell will ever be big enough, ever.

There are no medium-sized trees in the deep forest. There are only the towering ones, whose canopy spreads across the sky. Below, in the gloom, there's light for nothing but mosses and ferns. But when a giant falls, leaving a little space ... then there's a race - between the trees on either side, who want to spread out, and the seedlings below, who race to grow up. Sometimes, you can make your own space.

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.'

Look at Satan. Created as an angel, grows up to be the Great Adversary. Hey, if you’re going to go on about genetics, you might as well say the kid will grow up to be an angel. After all, his father was really big in Heaven in the old days. Saying he’ll grow up to be a demon just because his dad became one is like saying a mouse with its tail cut off will give birth to tailless mice. No. Upbringing is everything. Take it from me.