Different kinds of darkness can be written within a novel. And each kind can bring a certain intensity to your book. One kind of darkness is the bloody kind. This is where a writer writes about murder in a fashion that is descriptive but only as descriptive as the writer wished for the reader to read.
Another kind is psychological, which is where the writer writes intending to play around with the mind of their characters. This is where a writer may write about mental disabilities or create a character that because of their mental state, may do things that are abnormal and morally wrong.
Then you have what I like to call the lighter side of darkness, which I know is quite funny to say. But this is when there is darkness but more subtle, such as loss of a job or a heartbreak. Essentially darkness is just my way of seeing the bad things that can happen in my stories. It is my way of describing the stuff that goes wrong or that is morally wrong that we writers may write about. How do you see the darkness in writing a story?