The concept of the monster has almost always been with us, from the mythical through to the modern. Derived from montrer (to show), they are representational models of types of behaviour, our darker selves, presenting those aspects of humanity that we simultaneously fear and desire. Taking aside mass-media references laden with moralising, we can understand it to simply mean something that shows difference and therefore is often viewed as dangerous.
Man-made monsters in literature (made in the sense that they are created by authors, and also come to creation within the text by human hands) also represent acts of drive and power. Mary Shelley's unamed monster in Frankenstein is created to be a "New Prometheus", to be better, faster and smarter. The intention is to improve, to strive for greatness.
Perhaps you could see the Dominant a monster, certainly they emphasise the parts of us which we repress because we are conditioned to not to behave in such a fashion. They operate through flouting norms, re-arranging convention and by the more traditional monstrous attributes such as a love of cruelty and inflicting harm. The Dominant contains both the will to be different and the sense of danger that resides in the type of difference.
Yet the submissive is also a monster, although perhaps not as obviously so. We are perhaps conditioned into thinking of them in terms of victimhood, which negates the all important factor of choice. The submissive chooses to wear the collar, to kneel, to offer themselves up to all kinds of abuse. What is that, but a show of force, of will and of difference? A wholehearted embrace of that which is feared.
The monster overrides fear, either by becoming it or by accepting it. By Dominance or submission.
Acts which society is afraid of, or does not understand are often deemed monstrous, meaning "evil", but perhaps it is the act of revelation itself which gives the shock value: out of sight is out of mind, after all. We present ourselves, whether Dominant or submissive or anything in-between, through our actions, our flesh and our words. Our adopted roles are monstrous because of their demonstrative nature. We are monsters of performance engaged in a little play of show and tell.