Beyo d Good and Evil is a very interesting and thought provoking read for sure. This is one of the aspects of Nietzsche that is easily overlooked by people who want to see him as simply nihilistic and destructive.

"For Nietzsche, the construction of the self is not a religious act, an obligation, or an act of submission to nature, as variously seen by 'moralities' -- it is an art form. In The Gay Science he says something to the order of 'One must make of one's Self a work of art, carving away something here, growing something there, repurposing some mass of unavoidable ugliness elsewhere to present a more pleasant view from the distance...' (I do not have a copy here, and I cannot find it online, if someone can give me the words...)

Edited to add

"it is only they [artists] who have taught us how to estimate the hero that is concealed in each of these common-place men, and the art of looking at ourselves from a distance as heroes, and as it were simplified and transfigured,—the art of "putting ourselves on the stage" before ourselves. It is thus only that we get beyond some of the paltry details in ourselves! Without that art we should be nothing but fore-ground, and would live absolutely under the spell of the perspective which makes the closest and the commonest seem immensely large and like reality in itself."

-section 78 of The Joyful Wisdom

A monster is one whose 'self' lacks 'art'.

Power may be the medium of morality, and its goal, but tasteless use of power is like tasteless use of any other medium. To see his aesthetic, you can look at his own artistic process, which he displayed over and over again by choosing mythological or poetic representations, or you can look at his critiques of other's work. Particularly, I think it is why he bothered to publish 'contra Wagner'.

He accuses Wagner's music of being an assault on the audience, brandishing its scale in a way that shocks the senses and bruises the organs, and of having too little consistency and comprehensibility -- winding an endless melody, rather than a theme.

In this context, I think the quote about monsters indicates there are aesthetic choices that we should restrain ourselves from making even though they would be effective. We should choose scale, elegance and consistency. If others' use of power lacks art, we should not simply confront them with more power, if that involves less art. We should restrain ourselves.

In particular, I think 'an abyss' is a sort of monster, the monster of complete cynicism and true nihilism -- the completely empty man that early 'beatnik' post-modernism seems to favor. There is always power to be uncovered by renunciation of boundaries, but pursuing an utter lack of restraining form leaves one 'powerfully empty', and perhaps incapable of recovering one's artistic nature."