Content that on the surface seems worthy of GuroChan happens all the time, but real life has that peculiar way of making stuff sad and gross.
Scat happens all the time and nobody bothers talking about it.
A few years back there was a scandal centered on a woman who had gutted a horse and then pleasured herself in its remains to "become one with it."
"Mass" rape and torture is often a characteristic of the odd cult that gets busted every several years.
True mass torture is, unsurprisingly, extremely rare, but in a technical sense the famous Holocaust of World War II was an example. The problem is that basically nobody finds that event sexy or otherwise akin to what we write about in here, and the same holds true of other historical mass torture and snuff scenarios.

I personally have a hunch that if humanity follows its long-term trend of exponentially growing in population, habitat size, technological sophistication, and lifespan, many of our fantasy scenarios are going to find their way into reality. In the future it isn't hard to surmise certain parts of the world might be populated with people whose primary risk of death comes from suicides and who consequently end up having very little fear of death and don't value human lives as much, including their own. And in all likelihood a time will come, however long it may take, when people can participate in gory activities and easily reconstitute or outright replace their bodies afterward. Thus I find myself attracted to sci-fi guro stories like "MS Topless" or "Repairs" due to their relative plausibility.

As far as the average person becoming a monster, I'd say it's not hard. Simply maintaining a global, more or less peaceful civilization might well be our species's greatest achievement of all, considering the violent and cruel tendencies of the typical human. Simply living in circumstances in which a person can act like a monster is usually a sufficient condition for him or her to become one. Take the Stanford Prison Experiment, the Salem Witch Trials, or any of the thousands of racist riots that have taken place over the years as examples. These are, again, things I see continuing for the foreseeable future, so if your fantasies include people mistreating one another you'll get plenty of fetish fuel going forward ;)