I don't think there's really a "limit", but there should always be a "reason", or an "explanation".
Writing something just to be despicable and mean... anyone one can do that, there's no real challlenge and substance to it though.
"There was a girl, she was raped and killed and thrown into lava, just because someone felt like it. The end."
That's something that, unfortunately, you'd read about in your every day life. As a matter of fact, just the other day I saw some fucked up news about a teenager who was gang raped, shot, and then fed to the gators. Like, what the fuck right? They might as well keep that shit to themselves, there's enough screwed up stuff going around.
Anyway... I wrote a story that detailed the killing of a child. Even though I thought that the "twist" was a bit clever, people were a bit concerned and bothered that it was very "real", and there was not much else to the story; basically the character was fucked up, and did something terrible, willingly.
In this sub at least people will want something else to keep them going and guessing, that's what I think.