First images from Euclid are in

A fun thing I like to do every so often is to try to break away from the natural notion that space has a horizon and that instead force myself to feel that it continues equally in all directions.

We’re naturally inclined to be ok with giant distances on the horizon. It’s natural to put more emphasis on that part of the world. Hold up your thumb to the horizon and notice how many things fit alongside it compared to your thumb help downwards against the ground.

On the surface of our planet the up direction isn’t usually interesting and the down direction isn’t even there. It is therefore quite horrifying (“fun”) to imagine space going down forever.

Yes. I like to look at the moon and think of it as being "down" and I'm the one at an angle. Rather than "there's nothing under me, just the ground" it's "there's nothing under me, just nothing forever."

Many years ago I read some sci-fi novel, and in it was a sub-plot of a warring alien species that started destroying anything and everything they came across in their travels.

The story went that their local system was in some sort of a dust cloud, so they had no stars visible from their planet. At some point, that cloud somehow dissipated. On the planet, one of the inhabitants bothered to look up one night, and it hated everything it saw. So the race developed a space program to go out there and destroy it all.

For some reason I think it was Adams' H2G2, but the tone of my recollection does not quite feel on-brand for those stories. Not sure.

You are right, it is from h2g2.

It's the planet Krikkit, which is a major part of Life, The Universe and Everything, the third book in the series.