If it's a fade-to-black sort of situation it could work, easily. Or, if instead of being written, it's referenced as a past event (even if it has a few details). I don't know why, but it seems like the vast majority of people are much more forgiving or dismissive of things that have happened in the "past" of a fictional universe, or in real life. If it's in the past and they don't have to read about it as occurring in the actual events of what they're reading, they don't think much of it. I see this happen time and time again with characters who would otherwise be some of the most disgusting, vile, meaninglessly violent people ever, but because this was their past--like 1, 5, 10 years ago--and not in the present, even though it's made clear they haven't yet changed--it's okay. They can talk about it, they just can't do it. If they do it in the present, then more people notice it. Another example is Salem in Sabrina the Teenage Witch, who would say the most outlandish violent things, but because it was all just suggestions, or stuff about the past, and now he's a benign cat, he wasn't taken so seriously as to destroy the show.
You're right about how the level of acceptance changes depending on the medium. Novels can get away with more compared to visual mediums. Books are often more violent than their cinematic counterparts, so I do think you can marry both gore and killing and get away with it if the gore is kept to a bare minimum and not glorified, if that makes sense. Saying "...his head exploded, and the mess painted the walls and her clothes" sounds less extreme than "...his head exploded, and pieces of his brain splattered onto the walls and on her clothes. Blood colored her white shirt red, and...".
How to get away with it according to site guidelines...I think that's another matter, because writing sites are much more strict than what I've seen for movie ratings these days. I'd contact them if possible, or wait to see if anyone reports the chapter for incorrect ratings.
There's also the option to simply mark the chapter for Mature and let readers know what the chapter contains. Then, if anyone does skip it, offer a brief summary of what they missed, or indicate that it will be referenced later on anyway.
If on the other hand this story in question is not on any writing sites and will never be, but you'd like to publish it one day, I think you're pretty much in the clear. Editors will let you know what is and isn't too much, and the inclusion of it will not be the reason they pass on it because those are easy fixes.