Demon Slave probably sounds cooler to the average German than the direct translation.

That too, but it's also a space management/text formatting issue.

The original title is 4 words. Two of them only take up 4 characters, making it 9 in total + 5 for the gratuitous addition of "slave" in Latin letters. That is a lot shorter than the 7 words written with 34 letters it takes for the accurate English translation (German would probably be something along the lines of "Sklave der Elitetruppe der magischen Hauptstadt" which is 6 words and 47 letters).

You would have to make the nice lady quite a bit smaller to fit all of that on the cover and still have it be eye catching.

Now someone below mentioned that the titular slave isn't demonic, so they should probably have found a more fitting word. I can't offer a good alternative, without knowing the manga though.