Perfect murder (fiction)

The perfect murder is a murder which benefits the murderer, but also has no negative consequences for him or her; usually, this simply means that the murderer is never caught. Several factors might contribute to the perfect murder:
The murderer has an impeccably trustworthy witness who provides an alibi, which no other witness contradicts.
The murderer had no apparent motive to commit the crime, and thus is not suspected by investigators.
The murderer does not retain incriminating items or leave physical evidence of his presence at the crime scene.
The murderer cannot be convicted for the crime owing to a legal loophole that the murder knew would make a conviction unattainable.
At no stage in planning, committing or covering up the crime does the murderer take another person into confidence on any suspicious or illegal matter.

The concept of the perfect murder is closely connected with detective fiction and often crops up in the whodunit and the locked room mystery. In the latter case, the murderer usually tries to make their crime 'perfect' by creating the illusion that there is no physically possible way they could commit the crime. For example, the victim is found dead in an empty room with its only door locked from the inside. The murderer, of course, has used some clever method to lock the door after departure, and a verdict of suicide means that foul play is not suspected.
The idea of a "motiveless" perfect murder is explored in the novel Strangers on a Train by Patricia Highsmith, famously adapted into a movie by Alfred Hitchcock. The scheme is for two strangers who both want someone dead to meet randomly and "trade murders," each doing the other's dirty work so they each have no discernible motive for their respective crimes.
A perfect murder could involve making it appear that the person one kills is oneself. The film The Whole Nine Yards features a perfect murder in which a hitman kills someone and blows up his body, making facial identification impossible. Knowing the police would check the body's dental records, the killer has a dentist insert dental implants to modify the victim's teeth to match his own.
A perfect murder could also be made to appear as an accidental or natural death.