A sex scene where you’re wincing every few seconds as you’re writing it is going to feel uncomfortable to the reader; the same is going to be true with violence. How graphic you get is going to depend on your audience and your own comfort level.
There’s a lot that goes into writing a fight scene. Knowing your characters is important. The world they grew up in, their background, their family, their language, tells the reader what kind of a fighter the person is, even their weapon of choice. Because I don’t outline, character sketches are important. I need to submerge myself into the characters in order to understand where they are coming from, how far they are willing to go to silence their anger. Character sketches allow me to build tension in a scene even before the trigger is pulled or a knife drawn. From King Lear:
That said, any given situation can change a character’s expected behavior, and you, as the writer, need to let it play out. It’s not always as I had imagined it. Sometimes my fingers get way ahead of my thoughts while describing a violent act and I have to go back and straighten out what I really meant to say. That’s what the delete key is for.
How much violence is too much? Fiction is about communicating intellectual and emotional ideas. For example, Iain Banks and Stephen King - two very different writers, both with a tremendous capacity for dark writing - write about horrific and weird events. Their characters respond with an emotion that you'd expect. This places violence in its proper context, while reassuring the reader that they are on a journey and haven't been thrown into hell for the sake of it.
Still, we have a personal choice as to how much violence—sexual or criminal—we are willing to read or pay for. I’m all for excitement, intensity, and thrills, but I stop reading when it’s too graphic.