Quick question... is it meant / bad if you destroy the reactor at the top left of the map at the beginning? Because I arrived there first after I escaped from the prison. And since you can destroy this, I thought so: Why should I not destroy it, I have it then easier. Therefore, I now ask myself the question, was it planned that the player can destroy this reactor?

It is planned, but there are pros and cons.

The prison searchlights and laser weapons will be mostly disabled, which is good because they can't attack you.
But those searchlights and laser weapons were also helping hold back the biters. This leaves you with less time to escape, especially in multiplayer.

Good luck!

Yes...
I already noticed that with the less time...
When I finally found the rocket silo, it was already destroyed. I guess that means I have to try again from the beginning...

Most likely restarting will be easiest.

You can still research and rebuild the rocket silo where ever you like, but it will be a challenge.