Well, I was reading the comments ( protip: don't do that
) and I just wanted to point out some things:
1) It is not demonic pride that makes the demon mages to seek combat as much as we see, like some people seem to think ... atleast that is not all of it. Notice how young Frieren reacts to Flamme sugestion of not confronting the demons head to head: she just asks about the "feelings of a mage" and Flamme just sort of agrees with it ( hinting that she actually feels the same ) , but point out that just going by those "feelings" will get you dead more often than not.
In other words, in this world, using mage powers apparently gives you certain "feelings" and one of the side effects of those "feelings" is a tendency to go for combat with all fangs out even when it would probably not be wise to do so. And by the looks of it this affects demons , elves and humans alike ( in fact , the only mage we see in this series that appears somewhat immune to this battle lust is Fern, that had training from hel ... Frieren and that was drilled in that magical combat = pain
). Demons just pull out all the stops when confronted with what they see as bipedal sheep ( aka humans ) ...
2) Related to this, it is not that Demons will never hide themselves. Again, just look at this chapter: those three demon stooges were also concealing themselves as well in the beginning ( when they were most likely fighting elves ). They just broke cover when they were sure that what they had to face was a half dead elf and a apparently weak bipedal sheep ( Ok, better stop with the Overlord references ) mage and they felt no need to restrain themselves ( you might even argue that they did that just to scare the human mage in leaving the elf behind ).
In fact, this chapter makes the case that concealing magical powers is not as a hidden thing as that ( or maybe not even that dificult ): Flamme does it, Frieren does it without any training, Fern passed the last chapters doing it and the three demon stooges apparently also did that. The fact that the demons seem to think it is uncommon, cowardice even, either is demon arrogance or just that, while it is probably not that hard to limit yourself, mustering the self control to do so in face to those "feelings of a mage" is the real limiting factor.