X is for the Unknown in Murder Mysteries

In mysteries, X often stands in for the murderer. In the show Person of Interest, the unknown factor is whether the "number" (or person) is the perpetrator or the victim of a crime. Is X about to commit a crime? Suffer a crime? Should X be stopped, helped, or protected? 

Agatha Christie has her detectives argue that in order to catch the murderer, the detective must understand the victim. What type of reactions and interactions did the victim invite? In reaction, is the murderer fearful, revengeful, desperate, angry, vain...? 

And some of the best mysteries have memorable murderers. 

*Spoilers*

From the Golden Age,  

Dorothy Sayers' Whose Body delivers a fascinating and cold-hearted murderer in The Dawson Pedigree. The fretful innocence of the victim--and how readily the community accepts the death--underscores the murderess's paranoid self-protection. 

The murderer in Ngaio Marsh's Light Thickens is inconsequential since the main "character" of the book is inconsequential since the victim is also inconsequential. The main "character" of the book is the play. Final Curtain, however, places the victim center-stage; he is an aging actor after all. His ego and self-satisfaction, his demands on family and constant will-changing account for the behavior of the murderer who mirrors him. 

Josephine Tey's To Love and Be Wise, like a few of her novels, involves no murder. Supposed victim and perpetrator of the investigated crime are one and the same, and the character of that person explains the double role.

Agatha Christie's Evil Under the Sun focuses almost entirely on the clever murdering couple. They attract all the notice. Christie, however, links their personalities to the victim's ultimately naive nature. She appears a slinky, manipulative tease, but she is actually, as Poirot states, "The eternal victim." 

None of the victims deserves being murdered.  The above writers were thankfully not disposed to excusing cruel and violent behavior. 

However, nearly all of them spend time presenting their victims and murderers as members of various networks from family to career: networks that evoke a strong and memorable response. Marsh indulged the most in random or pointless and tawdry murders (that is, realistic murders). But those random, pointless, tawdry victims and murderers are far less memorable. 

A classic murder, it appears, requires a definitive X.