Hello!
My biggest gripe with this story is that, although its supposed to be a horror story, it just isn't scary. But why is that? Well, fear is a really difficult emotion to evoke. To truly frighten someone, only by using the power of the written language, you must master the techniques of storytelling.
The most fundamental of which are pacing and tension.
Pacing is basically, the rhythm at which the story unfolds. If the story has a natural pacing, then the characters are given enough time to develop. There are enough tail scenes (AKA character development scenes) to balance out the head scenes (advancement of plot). With a balanced structure, the story neither feels too rushed (as in, OMG Way too much action, so much going on, there's no time to breathe) OR too slow (yawn, too much character development but not enough plot.)
Pacing is a good storytelling concept to get a hold of grasp, but you also need to learn about tension, because they go hand-in-hand. What is tension? Tension, or suspense, is when the audience is wondering what happens next in the story. It keeps us engaged, and wanting to read more. The author can most easily create tension in the story by withholding information. In order to do this, you have to slow the story down.
Well... you don't make a good use of tension, and your plot feels too rushed.
If you want to learn more about tension, please take a look at this video. It's a highly educational review of the Hobbit and why the director didn't make a good use of tension. By comparing the scenes in the Hobbit with those in the Fellowship, you can clearly see the difference. Now, I really recommend viewing the entire video from start to finish, but for your convenience, I've skipped to the relevant clip that goes over what tension is.
Now let's take a good, in-depth look at one of your scenes, and see where you could have improved the pacing and tension.
Helen and Richard had been married for a few years when she accidentally walked in on Richard chopping up a corpse in the garage of their first home.
Okay, so in this introductory paragraph, you clearly establish what is about to unfold here. Now, I'm asking: "How does the wife react to witnessing her husband committing murder?" That's good.
He stepped toward her. "Helen, listen, I..."
Tears streamed out of horrified eyes and down blood-speckled cheeks. She left the garage sniffling.
And she witnesses the deed. Now, if I were line-critiquing this for grammar, I would have a huge gripe with the line where tears stream down horrified eyes. This is a major tell, and doesn't really allow the audience to assume how the wife feels. Like, it says, right there, she's horrified. Not the most creative way to go about describing a reaction, but... moving on.
He found her in the bedroom, crying in their bed with a Bible clutched against her heart. Out of the suit, he sat on the edge of the mattress and confessed.
Okay, now they're about to have a confrontation. This is a good chance for you, the author, have an emotional moment.
But ... the your dialogue breaks all possible tension.
"Ever since I was a child, I've felt The Urge," he said. "It came to me when I was 5 years old. It started with small animals. I'll spare you the details, but those are the only kills I still feel guilty about. Then I moved on to people. I used to feel guilty about them too, but I've been doing it so long I've just gotten used to it."
IMMEDIATELY, the husband just... tells his entire life story! First off, this is straight up exposition. This is the equivalent of having the girl narrator from The Last Airbender movie tell us everything that's happened.
The main issue with exposition is that it's boring. It's most likely going to be boring. This is bad. And here, well, it's definitely boring. Turns out that even if you hide your exposition by burying it in dialogue, it still isn't entertaining.
But. Onto the tension.
What does this dialogue accomplish, pacing wise? Or to be more accurate, what big opportunity did you miss here, that could have made this scene great?
You didn't create any tension.
The wife sees the husband commit murder, and flees to the bedroom, sobbing. The husband then shortly comes to her room to apologize and to confess. He sits on the bed and they have a conversation.
Would he really just spill his entire secret like that? No. He would (most likely) have tried to hide his secret. What if the wife demanded an explanation, right there, and the two have a discussion where the husband is clearly hiding something, and the wife is trying to pry information out of him?
That would possibly make for a scene with conflict and tension. The husband is hiding information from his wife, but he also wants to reassure her that she will be safe. Meanwhile, the wife is potentially scared for her life, unsure that if she pries too deep, her husband will eliminate her as a witness to testify against her, but she also wants justice.
In this imaginary scene, both characters have separate goals to achieve, with consequences if they fail. And the longer that the husband goes without confessing, the longer the tension builds. And the more tension, the bigger the eventual climax.
But here, nope. The husband just goes right ahead and confesses. He doesn't try to sidestep the situation, or prolong the confession.