I don't do this for shock value (which has also been suggested by one particularly hostile reviewer back in the day) but because it is my style. This is how I write. I don't exactly sit in front of my laptop and consider how I can shock people, but I instead follow my writing instinct. Admittedly it sometimes takes me down a dark, twisted, and even questionable road, but one thing can be assured, I'm never bored. To me, this is a good sign. If the writer isn't bored, chances are good the reader won't be either.
The truth is that only in very extreme situations is a writer going too far. In the end, a writer must always express themselves and when writing fiction, must also tell the story through the protagonist's perception. It isn't for us to censor our characters (within reason, folks) but to tell their story with as much honesty and authenticity as possible. As a writer, I have the freedom to express myself as I see fit. As a reader, you have the right to choose what you do and don't want to read. It's as simple as that. 😉