Human Remains, by Elizabeth Haynes, is classified as a mystery novel, but from nearly the beginning there is no mystery about the identity of the man who has been "killing" people in the protagonist's neighborhood for years. Since it does not appear that you have yet read the book, these questions will give you something to think about as you read it.
First, why do so many people die without anyone really noticing that they are gone? When you can answer this, you will know what the victims have in common--and probably who the next victim will be.
Second, is convincing people to do something that will kill them the same as actually killing them? The antagonist claims he simply gives his victims "a push" in the "right direction." (Note the word killing is in quotation marks above, since that is one of the questions to be answered in this novel.)
Finally, what responsibility do the neighbors, families, and friends of the victims have for the victims' deaths? It is clear that fewer people would have died if the people who claimed to care for them had been paying more attention to them, so can a case be made that these loved ones are partially to blame for the deaths?
This novel is a study of the mind, sometimes called a psychological thriller, so it is important that readers ponder these kinds of questions as they read rather than just try to figure out "whodunit."