Hey Anon,
Getting into a dark mood for whenever you need to write can be a pretty tough thing to do. Adding on from what Elizabeth (above) has mentioned to get the creative juices flowing, I think understanding the 'vibe' you're trying to give off is extremely important. Most of the 'vibe' you get from characters, narrators, and so on, are usually linked to the voice you use in your writing.
Although in films this can be helped with literal voices, in writing this must be accomplished by manipulating your word choice and structure. This can be done by metaphors, a sinister setting, or even changing the length of your sentences.
For example:
1. "He had no choice. This is what he had to do."
2. "He had exhausted all his options and so came to the conclusion that this was his only choice, so he had to do it."
Essentially both say the same thing but they both give off very different vibes. The first sounds dramatic, whereas the second was more boring and logical.
I find, the best way to achieve a desired voice is to really understand your character/narrator's motivation and let the sentences flow in the way they would think internally. This way the inner voice of your character is shines through the external narration.
Hope this helps,
Abdul