I'm seeing that this is not a popular movie, and there is a reason for it, but if someone has watched it and left with questions I hope my comment helps.
I've read the original novel which was a masterpiece, and this adaptation is just horrible. Let's do a comparison.
In the original story, Wakatsuki has some mental issues, due to his older brother's suicide committed at a young age. He has been blaming himself since it happened, and he sees the same nightmare about a spider holding his brother hostage and chasing him.
This issue is the reason for telling the lady about considering suicide once more, and this is the reason he couldn't get his thing stand up. None of these things were explained.
He has been with his girlfriend, Megumi since university in the novel, and they just randomly place her in the movie.
The lack of explanation is one of the biggest issues of this adaptation.
Another big issue was the mood-setting, which was terrible. The background music was really bad, destroying the mood. The color choices for Sachiko's clothing were terrible. They did a bad job of portraying Sachiko's personality.
Bowling was never once mentioned in the novel, however, they decided to add it, and it was so meaningless and again, destroying the mood. I'd rather not talk about the random swimming scenes.
And the unreasonable erotic scenes especially in the ending it was ridiculous. (There were none in the novel luckily)
The original story wanted to emphasize the idea of Japanese society sinking so deep that they willingly commit various crimes for money. The novel also questioned the incompetence of the Japanese police (which was not shown here).
The director went to a classic horror movie rather than grabbing the meaning of the book. Kuroi Ie by Kishi Yuusuke was one of my best reads, and my hand was shaking in fear while reading some scenes, but this adaptation is nothing more than a huge disappointment.