These stories are human evil.
In my own writings, I am often drawn to cthulian, lovecraft style evil. It's partially because I have a strong science fiction and fantasy background. Don't get me wrong, of course King does this too, but that is not the type of story reflected here.
Here, there are murderers who are twisted parodies of humanity. We are taken deep into the mind and understandings of people who lust after pain and death the same way you lust after cheetos and hamburgers. You aren't even halfway through the first story before your boundaries are being pushed, before you are ever so-slightly squirming at the depravity of the person whose viewpoint you are rushing along with.
Yet, you cannot stop.
As the stories progress, you begin to find small connections between them. More than just "theme" it is completely possible that some stories share characters, even though we aren't told this directly. It's difficult to even think about, however, because you are so drawn to the visceral horror of the piece.
When madmen are protagonists, the result is a lot of fun.
I didn't even have to "evil up" this review. The book does it on it's own. It is a twisted, guilty pleasure.
Try it. You will not be sorry. I promise.