Never One Murder

It will have blood; they say, blood will have blood.

—Macbeth, Macbeth

Episodes of detective series are rarely happy with just one murder. There has to be two or three, with the second being found just before an ad break. Frequently the second murder only confirms the detectives' suspicions, especially if the original murder was disguised as a suicide or accident. If the prime suspect for the first murder is the next victim, it's Suspect Existence Failure.

Subtrope of Crime After Crime. Murder Is the Best Solution can lead to this outcome.

As a Death Trope, Spoilers ahead may be unmarked. Beware.

Examples of Never One Murder include:

Anime and Manga

The Kindaichi Case Files, and most of the time the killer was targeting all of his victims.

Film

Forms the plot of the movie Very Bad Things. The first death was an accident. The protagonist's attempts to cover it up lead them into a string of murders, eventually reaching the point where they start killing each other, afraid that one of them will talk.
The movie A Shot In The Dark has several murders, though many of these are botched attempts to kill Inspector Clouseau. The play it was based on had only one murder.
Marv's story in Sin City starts off with just one murder but it is soon revealed that the Big Bad is a Serial Killer who has had many victims. Similarly, Marv is "killing his way to the truth."

Literature

Agatha Christie's detectives (particularly Hercule Poirot and Miss Marple) would often posit that once someone has committed a murder for a dire reason, it becomes easier to kill again (often to cover up the first crime), and then increasingly easy after that, until the killer is acting on the most trivial of threats or slights.
- In The ABC Murders, a mass murderer hides the murder that would benefit him amongst a series of others, creating an ABC murderer. "When are you least likely to notice a pin?" "When it's in a pincushion!"
- This is also lampshaded in the book, where Poirot and Hastings discuss detective stories, and Hasting says that more than one murder makes them more exciting.
- Also happens A Murder Is Announced, where Charlotte, who's been posing as her sister Letitia all this time murders her best friend and another one of her friends for coming close to discovering that she was responsible for the first murder of the man.
- Other examples include Cat Among the Pigeons in which there are three murders committed by two people with entirely different motives who are not working together and The Clocks in which a second murder is committed to help cover up the first.
Raskolnikov in Fyodor Dostoevsky's Crime and Punishment kills the miserly pawnbroker as per his original plan, and then, without coming to his senses, kills her younger sister who just happened to walk into the room. This is used to support the Aesop.
Crime writer Harriet Vane describes this trope in the Lord Peter Wimsey novel Have His Carcase:

'No; well, there's the Philo Vance method. You shake your head and say: "There's worse yet to come", and then the murderer kills five more people, and that thins the suspects out a bit and you spot who it is.'
'Wasteful, wasteful,' said Wimsey. 'And too slow.'

There's a particularly unpleasant variant in the G. K. Chesterton Father Brown story The Sign of the Broken Sword. An army general murders the major who discovers he's been selling secrets to the enemy, then covers it up by leading his unit into a battle he knows they will lose, to create more bodies.
- Subverted with poetic justice in that the surviving troops figured out what he'd done and strung him up immediately after the battle. Alas, they kept it secret, so his reputation stayed cleaner than he deserved.
  - Well, depending on how cynical you're willing to assume the soldiers were, they might have done it to keep from getting executed; any evidence they could have presented that would prove the officer's wrong-doing may well have been discounted out-of-hand.
Rita Mae Brown's long running mystery series, Mrs. Murphy Mysteries involving Harry Harristeen and her pets always follows this. At some point in the first 50 - 100 pages someone will die. Then two or three other people. It is always easy to narrow the list of people going to die because they are appearing in 'cozy' Crozet, Virginia for the first time.
A common occurrence in the Kinky Friedman novels. Often Kinky is implicated, not always, but often.
In Donald Moffitt's Short Story A Snitch in Time (published in Analog SF and F ), the cop hero, Delehanty, is killed by the murderer he's been chasing, which finally results in the murderer being caught.
In Death: This trope is used many times throughout the series. Memory In Death and Fantasy In Death are the few expections. Sure, both stories had the murderer try to kill someone else, but the victim survived.
Shows up a few times in Henning Mankell's Kurt Wallander novels.
Jo Nesbø's Harry Hole novels are fond of including serial killers (or killers that appear to be serial killers), so naturally this trope shows up several times.

Live-Action TV

Taggart is notable for this, with only two or three cases of this not happening, as is Midsomer Murders.
This one happens a great deal in Monk, but "Mr. Monk and the Actor" gave it an ironic twist. A character asks if the murderer is likely to strike again; he's told that it was a crime of passion, and the man who did it will probably never put another toe out of line. Cut to the murderer breaking into a pawnshop, which leads to his second murder.
Watch an episode of The Inspector Lynley Mysteries, Foyle's War, Inspector Morse, or Lewis. Just about any episode. You cannot escape this trope in recent British mystery/crime and punishment series.
In Days of Our Lives, the Salem Stalker killed one woman (who was going to marry the man she loved) and was forced to keep killing people in order to cover up the murders. Naturally, another villain took the opportunity to bump off her husband, figuring she'd frame the serial killer for that attack. The cops weren't fooled.
In a skit for Children in Need, DCI Burke and DI Rebus (both detectives from ITV) meet at a motorway service station halfway between Glasgow and Edinburgh, where a man's been murdered. Rebus' female detective companion expresses her concern that there might be another murder. Burke's response.

We're on The BBC. We don't need to kill people just to keep the audience watching after the ad break.[Words to that effect at any rate].

Midsomer Murders has been a frequent user of this trope up throughout its long run, though in the last couple of years, we've actually gotten through an episode here and there with "only" one stiff.
New Tricks likes to play with this. A number of cases where we are led to believe follow this trope, turn out to be just one murder or even just a single accidental death. Other times the murders are related but perpetrated by different people for different reasons.
Nearly ever episode of Criminal Minds deals with a serial killer.
In the Supernatural episode 'Usual Suspects' in which the boys get arrested for the first time, the killer in the case turns out to be this, a crooked cop covering his tracks, and the supernatural being they were hunting turns out to be his first victim trying to warn the rest.
Happens in several episodes of Numb3rs.

Theater

Older Than Steam: This is most of the plot of Macbeth. Macbeth murders Duncan to become king, and finds himself forced to commit more and more murders for self-preservation.
- Christopher Booker's understanding of the Tragedy plot, well displayed in Macbeth, includes three murders: the Good Old Man, the Rival (or Shadow), and the Innocent Young Girl. Each has symbolic significance. The first murder tends to lead to the others (downward spiral into worse crimes) and the fate of the Girl tends to seal the fate of the tragic hero (ultimate destruction). (It's possible to avoid direct murder of her: violation works, as does driving her toward insanity or suicide.)
Arsenic and Old Lace: A body in a window seat turns into twelve as the hero discovers that his sweet, innocent aunts have been quietly murdering lonely old men for years.

Video Games

The fifth case of the first Ace Attorney game gives us the Joe Darke Killings. The titular serial killer accidentally ran over a woman with his car, then killed the two people who witnessed the accident, followed by then the kid who saw him burying the bodies, in order to keep the whole thing quiet.
- And in Trials and Tribulations the killing spree spanned six years. First Dahlia arranged the suicide of Terry Fawles, killed her own half-sister, then poisoned Diego Armando (he got better) and electrocuted Doug to keep them quiet. If you include Terry this gives her the highest body count in the game.
The white chamber: The killer aka the main character killed her first assistant by accident and hid the corpse, then killed the engineer when he found the body, then murdered everyone else one by one out of sheer psychotic paranoia.

Western Animation

Parodied in the first "Anthology of Interest" episode of Futurama. The "What If" machine shows what would happen if Leela were more impulsive: she murders the professor and then kills nearly all the remaining cast members to cover it up—except for Fry, who she keeps quiet in another way...