Like some here, I actually have a similar fear for one particular part of my comic, to the point where I've been flip-flopping on whether to try and avoid it or not, this coming from someone who's comic has had throat tearing and popping eyeballs. XD
However, that also means my comic has had some very messed up scenes already and I've never received any real backlash, even though the comic started out pretty tame. I can only trust people to turn away and stop reading if the comic becomes too uncomfortable for them.
You can say "there will be dark stuff in this comic" or "tthere will be violence/trigger fuel/ect" if you feel the need to be more specific, I don't think either of those type of warnings will really spoil the flow of the story. Then at least you can say you warned people.
I doubt any shift in the story's mood will make you lose tons of your regular readers, I wouldn't worry about it, but either way, sometimes you just have to go with your gut so you can tell the story you want to tell and not worry so much about people's feelings. Being considerate is all well and good, but worrying about that too much can also drive you crazy and lead to making edits you don't really want to make...then suddenly you've lost your original vision.