It's a fine line. Are you writing darkness to introduce conflict and grow your characters, or are you writing darkness for the sake of darkness?
I often feel self-conscious when people ask me what my (current) book is about; particularly family. Especially if I get past the two-second pitch ('it's a normal girl vs. criminal underworld kinda story') and they want to hear more. 'Well, Auntie, there's a kidnapping, a fair amount of bloody violence, some language, and a considerable number of unsavoury characters who always seem to win... sounds great, right? I'm targeting it at young adults, like my cousins."
I think most writers must feel that trepidation at some point: that people will judge them by what they've written in their (fictional) book. But stories are NOT often about good people being good. There's nothing interesting about someone who has no problems. By most people's definition, a story begins with a problem, or a character that is lacking something. Through some kind of trial they grow and/or overcome it. The problem has to be big enough that it is a challenge for the main character (which is where you run into difficulty telling a story if your main character is, say, Superman or The Vision, but I digress).
Ok, so you've established you need a problem, and one serious enough that the character will have great difficulty solving it. This is where you might find some pretty uncomfortable shiznit, depending on the style of story. Suddenly what you've written becomes something you'd rather not bring up at family gatherings.
That does not necessarily mean we writers have unhealthy fixations with death, dismemberment, torture, etc. It means we feel the character needs something very serious to challenge them, and hopefully, put readers on the edge of their seats.
But writers write from personal experience, you say. Yes, we do. We draw emotional reactions and moments from things we've experienced. We use that to hypothesize the thoughts and feelings of characters doing things we would never do. It doesn't mean we want to do those things - it means we want to write a good villain, or a good challenge, into our story.
My rule is: write darkness because darkness makes the brightness of your character's success seem brighter.
You see? Just because I write some very sadistic, hostile characters into my story doesn't mean I'm mentally unbalanced...
...Or does it? O.o
Also - the attached artwork is a cover I drew for a comic written and produced by Higher Universe Comics, for a story called Mental Case. Higher Universe is an Edmonton-based comic company, check them out and support Albertan creators!