Suspect Existence Failure

Somebody has died. Often, several people have died. The evidence all points to one particular man (and it usually is a man). Everything implies that he did it, and the case looks almost over. Except that then something awful happens: your prime suspect dies. Only by dying could he prove his innocence.

Often it's the actual culprit who commits the murder. If the killer knows that his victim is a suspect, and the murder was avoidable, this is obviously a bad move on his part, as it usually shakes the detectives out of their complacency and forces them to look at other suspects, including the actual killer, anew.

This can be done in the case of other crimes. They almost always are serious crimes, however, since otherwise upping the ante to murder is a fairly stupid risk. Killing somebody who caught you stealing from the cookie jar is right out, unless it's the end result of a steadily escalating stream of cover-up crimes.

A standard way of revealing that a certain avenue of plot is a Red Herring, and is fairly common in Cop Shows, Spy Dramas, and Thrillers, although TV series tend to be fairly careful about overusing this particular trope, or they would lose the audience's trust. Often used as the reveal when Your Princess Is in Another Castle. Suspect Existence Failure victims often have a habit of popping up as Peek A Boo Corpses.

A common subversion, of course, is that the Suspect actually is the killer, and was faking his Existence Failure all along. Another possibility is that the Suspect was the killer you were looking for, and just happened to be killed by someone else (perhaps using the same MO, trusting that his solid alibi for the murders he didn't commit would keep him from being suspected for the one he did).

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

Examples of Suspect Existence Failure include:

Film

Bizu in The Pink Panther.
In Scream, Billy asks, soulfully, what he has to do to prove his innocence. A second later the killer leaps into the room and stabs him. Because Scream never met a trope it didn't want to play with, it turns out Billy's the killer anyway. There were two killers and they engineered the whole thing to screw with Sidney.
In Chaos Theory, a cop gets blown up just as fishy smell hits the air. In the usual subversion, he is not actually that dead in the end...
Agent Kujan from The Usual Suspects tells the same subversion story about Dean Keaton. Some time in the past, when Keaton was among suspects for some crimes, he dies. Then, in few months, witnesses die too, someone else gets convicted, and you guess it, Dean Keaton turns up quite alive and well. This episode is presented by Dave Kujan as a pinnacle of Keaton's evil, so they appear to be quite genre savvy in that regard.

Literature

In a A Study In Scarlet, Lestrade is prepared to arrest his suspect, only to find that he's already been murdered.
The trope occurs several times in Agatha Christie's And Then There Were None. Ultimately subverted and then played straight, as the murderer faked his death precisely to eliminate himself as a suspect. Of course, then its ultimately played straight as he then commits suicide to complete his perfect crime and, again, eliminate himself as a suspect.
In Death Note: Another Note, the Los Angeles BB Murder Cases, Rue Ryuzaki, a.k.a. Beyond Birthday, plans to create a case that L can't solve by committing suicide and making it look like a murder. Up until this point, he's been portrayed as the prime suspect.
- YMMV on him being the Prime suspect. Naomi herself notes that while creepy, he doesn't appear evil, and We're led to believe that it's L, not Beyond Birthday
In The Ninja, the antagonist Saigo uses the fake version of this to throw law enforcement and the antagonist Nicholas off the trail, killing a man of very similar build and chucking him off a high tower block face first to fool identification procedures. (The novel is set in 1980 before routine DNA analysis).
In Death: This trope popped up in Innocent In Death. After Craig Foster's murder, Eve Dallas ends up focusing on Reed Williams as the prime suspect. Take a wild guess on what happens next.

Live-Action TV

In the Lost episode "The Other 48 Days", Nathan was suspected of being a spy from the Others. He was murdered by the real spy, Goodwin, who hid the body and let the other survivors believe that he had escaped. In this case, the action was justified in the dialogue, as he was worried that further interrogation from Ana Lucia would eventually prove that she had the wrong guy.
Done repeatedly in Forensic Drama series such as CSI and Bones.
Not used as much as one would suspect in the various Law and Order series, but one notable example from the SVU series involved the demise of a rapist whom everyone was convinced had killed his victim. His death started them thinking that her killer had an entirely different motive.
Subverted in an episode of The Persuaders! where Brett's relatives are being murdered: all the suspects die... then it's revealed that one of them was actually the murderer, and faked his death because he knew that would clear him.
- Done similarly in a Ten Little Murder Victims episode of Remington Steele.
- Ditto for a similar episode of the television program The Wild Wild West, "The Night of the Tontine".
- And the John Steed/Emma Peel The Avengers. At least once.
- The Adventures of Brisco County Jr. has a Ten Little Murder Victims episode with a gathering of Bounty Hunters at a secluded hotel.
In The Twilight Zone episode "Shadow Play", Dennis Weaver is on Death Row trying to convince people that the world is a nightmare he keeps having, night after night, over and over again. The Warden is finally convinced he must be insane, and calls the Governor to ask for a stay. The stay comes too late, the electric chair is fired up, and we find he was right: everyone else dies, and his nightmare starts all over again.
Canadian comedy team Wayne and Shuster did a Sherlock Holmes skit in which everyone Holmes accused of being the murderer would then be murdered in a different way, in some cases a quite ridiculous way. When he finally got down to accusing the butler, a B-52 flying overhead dropped a load of bombs, and the butler was crushed by rubble. Then Holmes realized the truth; as he told Dr. Watson: "We're in the wrong bloody house!"
One episode of Psych had Shawn suspect a fashion mogul of killing her husband right up until she died at his funeral. He then commented "Okay, probably not the wife." Ultimately subverted when it's learned that she actually did kill him. She died from the delayed effects of him poisoning her.
In the Buffy the Vampire Slayer episode "Killed by Death", the Scoobies become convinced that the villain killing the sick children in the hospital is the creepy (and, it turns out, criminally negligent to the point of malpractice lawsuits) Doctor Backer... until Backer himself is killed by the invisible monster that is actually killing the children.
In Sanctuary's first season episode "Kush", there's a crash, followed by several murders. When they settle things down by supposedly catching the suspect, he's killed and they have to reexamine their 'fool-proof' method of determining the killer.
The Columbo episode "Last Salute to the Commodore" starts out as a normal episode, with us seeing Robert Vaughn disposing of the victim's body and setting up his alibi, Lt Columbo comes in and starts his usual harassing of the suspect, and then half way through Robert Vaughn turns up dead, and the episode suddenly turns into a Whodunnit.

Newspaper Comics

Gary Larson drew a The Far Side strip that had the caption: "Blast! Up to now, the rhino had been my prime suspect!"

Video Games

A Garrys Mod gamemode named Trouble in Terrorist Town can have this happen as a traitor kills a suspect or even more likely the innocents killing off a suspect who was actually a innocent.

Western Animation

Parodied in Futurama, "Anthology of Interest", where Zoidberg's summation is repeatedly interrupted by this. Further twisting it is that the new victims each figure out who the real killer is seconds before their deaths (which is why they get killed).
Unnecessarily lampshaded just before the end of Robot Chicken's "Michael Jackson vs. Michael Jackson" sketch.