Since we are talking about fiction, no story is a story without a hero. And since a dystopian society is inherently evil by nature, something we abhor, even if the hero may not, the hero must be a victim of the system in some way or another. Initially, the hero may not even realize he is a victim of the system, but we do, or perhaps we learn how as he learns it.
It doesn’t have to be in a huge way. It may be ‘a day in the life’ of sort of story and the hero is fighting just to get his next meal, but the system itself has to be something that impedes his action, something that unsettles him and ultimately persecutes him, and something that, in the end, he rejects.
In The Ogress’ Son, the system is what causes Slade to go on a vengeance quest in the first place. He can’t appeal to local law enforcement or the local lord, and under those circumstances, he decides he has to take his own justice. He’s unwittingly fighting the system itself.
There aren’t too many scenarios where vigilantism can be justified, if ever it can be justified. Wrapped in anyone’s concept of statehood is the idea that the state exists for the support of the common weal. As a vigilante takes it upon himself to mete out justice, and pit himself against the natural order (i.e. the purpose of the state), interference in the functions of the state are a result. If it were allowed, it would be impossible for the common weal to be protected and supported.
In a state that is inherently disordered and lawless, or conversely, a state that removes the autonomy of humanity with the justification that it is for their own good, while also failing to provide a way to protect its citizenry, as is the case in many a dystopian, I would argue that vigilantism ends up being an acceptable means of recourse for justice.
Certain people liken it to the wild west. I suppose they aren’t wrong, but autonomy and not being victimized by the state is a much surer way to a stable society than its opposite ever will be. But stability and peace do not make for good drama. Fiction requires conflict. Thus, dystopian fic, which pits a perverted state against a given individual or group of individuals, will remain a perennial favorite for examining the human condition through fiction.