How to Write a Dark or Sad Story
When you're writing a dark or sad story, you'll want it to be as realistic and engaging as possible. Try listening to sad music and dimming the lights to set the mood for your writing session. You can also do research about whatever you're writing. For example, if you're writing about someone with cancer, read other stories about it or watch vlogs from people dealing with cancer for inspiration. Then, start to plot the main events of your story in a rough outline. Don't worry if your story changes while you write it, since the outline should only be a guide. While you're writing, try to put yourself in your character's shoes and imagine everything they think and feel.
Get an outline. List the events of the story, and figure out the plot line.  It's best to know what you're writing about so you won't have to worry about it when you're trying to form the words. As your character goes through the bad events, come up with detailed images in your head. Using your creativity, form words to go along with that image. Wait for a day when there is heavy rain. Once the raindrops start hitting the streets, go outside and walk around. This way, you'll achieve a sense of seriousness, a dramatic feeling, or inspiration. If you would prefer not to get wet, carry an umbrella with you. Read some dark literature. Sometimes, seeing how other authors wrote their stories may help you write yours.  Don't be afraid to try out their writing style, but make sure you aren't plagiarizing. Get a feel going. Play a very sad or depressing song. Often, music will unlock a feeling inside of you that cannot be brought out in any other way. You could also construct a playlist of depressing, sad songs so that you have a continuous loop of them playing. Write in a place where you're all by yourself. Distractions often can divert your attention from writing the story. A loud, noisy environment where you cannot concentrate makes it harder for the words to come out, and you'll only end up with frustration. A quiet room is what you'll need in order to properly think. Try to get the feel of what you're writing about. If your main character is a patient with terminal cancer, then go visit someone who has terminal cancer. If you can't find one, go online and look up some journals. Dim the lights when you write. You don't want any light to blind your sight into the words of your mind. This will also create a dark atmosphere in which you'll truly feel depressed. Write as if you're taking all the sadness, desires, hate, frustration, and hurt in you and turning them into words. This can be immensely cathartic. Use a thesaurus if necessary in order to come up with heart-touching words. Use good punctuation. Full stops at the end of sentences. If you're not sure about the commas, have someone with the eye of an eagle and the heart of a grammar Nazi look it over for you. Don't overuse ellipses. If you are taking in all the sadness of the world, you will be mocked beyond your comprehension. This act takes over your life. If you really want to save the world, or give the world resources to do such, ignore the world's mockery and resentment and write to the best of your ability. 
Get an outline. Wait for a day when there is heavy rain. Read some dark literature. Get a feel going. Write in a place where you're all by yourself. Try to get the feel of what you're writing about. Dim the lights when you write. Write as if you're taking all the sadness, desires, hate, frustration, and hurt in you and turning them into words. Use good punctuation. If you are taking in all the sadness of the world, you will be mocked beyond your comprehension. 