Murder is a violent act. It's the taking of a human life. That goes without saying, of course, but if you think about it, something like a murder, which is a violent act, has to have powerful effects, not just on the victim, but on the people in his or her life.
Those effects - the aftermath of a murder - are, I think, an important thing for a mystery novelist to consider. If a murder mystery is going to be authentic, it should address what happens to the people involved in the murder. Agatha Christie dealt with this in several of her novels. And Then There Were None is a particularly powerful treatment of what happens to a group of people when, one by one, they are killed. The changes in the ways the characters deal with one another make the novel unforgettable, even if one weren't interested in finding out the truth behind the deaths. In Ordeal by Innocence, we see how each of the members of a family deal with the death of one relative when an "outsider" tries to prove the innocence of another.
Perhaps the most interesting exploration of the aftermath of murder in Christie's books is her treatment of it in Murder in Retrospect (AKA Five Little Pigs). In the novel, we learn about the murder of Amyas Crale, a famous painter, through the eyes of the five people on the scene the day of the murder. What's even more fascinating is that we see what happens to those people after the murder. We see how being involved in a murder has affected them, even years later, and how each of them reacts to it.
Christie isn't the only one, of course, who's dealt with the question of what happens after a murder is committed. Ellery Queen's Calamity Town focuses on the effects on a wealthy, middle-America family when it becomes clear that one of them has poisoned the sister-in-law of the youngest member of the family. I recommend Calamity Town, actually. It's an interesting study of family relationships and how they're affected by sudden death.
In my own Joel Williams series, I try to keep in mind the enormity of murder as I write. In Publish or Perish, a university department is rocked, and all of its members affected when a graduate student is killed. In B-Very Flat, the death of a musically-gifted undergraduate student changes the lives of her roomate, her family, her partner and her musical rival, among others. In a good mystery, it's not enough to simply tell what happened (the murder). It's also important to think about the effects of that murder. The aftermath of murder is as important to the story as the murder itself is. It adds believability and interest to a novel and after all, I don't know of any novelist who doesn't want her or his work to be interesting and believable.