In the ranks of Demon Society it is completely possible to be a "masterless" demon. You need bow down, and answer to, nobody.
However the Masters control all the gates to the Human World. Without access to the Human World, a masterless demon has to find food and resources in the demonic planes.
However, access to food and resources is entirely controlled by The Masters on the demonic plane.
This usually results in the Demon withering down to an imp, or else fighting and winning enough times to be of interest to a Master, and then (ironically) being actively recruited.
So maybe it is better to say that it is completely possible to be a "masterless" demon, temporarily.
-Notes by Demon Student, Jahi