Three things are likely to happen when you write about the things you will not, or cannot, say out loud: You get to the truth of your story quickly; your characters become more human and multidimensional; your readers will believe even the most outlandish of your fictions.
The boogeyman is not confined to the genre of horror fiction. Your story can be about a great love, a great escape, or a great regret, with nary a bloodsucking alien or monster in sight. When your hero or heroine is propelled by fear—the fear of losing someone or something, the fear of failing, the fear of succeeding, the fear of being found out—you increase the sense of urgency in the plot line. Your "pretty good" story becomes a compelling page-turner when you invite the reader, ever so gently, into the claustrophobia-inducing rooms of your protagonists' fears.
As a writer, you also have a special license to talk about the things the rest of us are too afraid (or too polite) to speak of openly. Upon hearing news of a tragic event—a plane crash, for example—the acceptable and expected reactions are to feel sorrow for all those who perished, perhaps to offer a small prayer, or simply to utter the formless sounds that express our horror and dismay in all the ways that words cannot.
But what if one of those who died was a torturer of small animals and children? What if one of those who perished was destined to die an even ghastlier death in another day or two? What if one of the dead was on a mission to rectify, at long last, a great wrong that will now remain unredeemed forever? What if someone on that plane was better off dead?
We tend not to talk about such things in real life.
But you can write about them.