As a blanket statement, any adult-rated MU* should be considered Adult-Rated (R or higher) in a fair majority of the areas, events and plots. Whether that rating is for violence, sexual content, or whatever. If the game states that you must be 21 or 18 to play or whatever, it's adult-themed!
Since the dawn of WoD, it seems, that people are routinely surprised by Evil Characters acting Evil, even when there is a lot of evidence (perhaps subtle) that they plan to do so. No, I'm not talking about surprise buttsex trauma, I'm talking about watching an IC event in a public location, with a timestop dropped (remember the days) and people reading a BBpost to get in... and someone complaining about the violence in the scene.
If you are on an Adult-Rated game and have ANYTHING that triggers you in very horrific ways, I would sincerely hope that you have some tidbit of information on you somewhere that warns potential STs, Staff or RP partners that even starting down that road has the strong possibility of killing your involvement, interest or whatnot. Even if that note is an OOC statement at the start of meeting a new partner in a scene, or whatever.
What I'm saying here is that: 1) Players should expect Evil, Twisted things on a game where Evil, Twisted lives and thrives. 2) STs and other PRP runners should not 'surprise plot twist' as @surreality says, at least OOCly. Give warning that Evil, Twisted things are not just possible, but imminent, here's your warning, please leave now kthnxbye.
This should be an ingrained part of our culture, don't you think? Doesn't this fall under Respect Your Fellow RPers? Maybe. Surprisingly, this is the EXACT reason why we need News files so explicitly and agonizingly worded.
It's the MU* equivalent of the sticker warning on your clothes washer expressly telling you that no, it is not safe for you to wash children in the machine, FFS.