If a character has special properties, other people usually want to know how to get or understand them, for reasons ranging from For Science! to world domination. This means that they need to figure out how the special properties are created, and how they can be replicated. That means they need a sample of the blood or genetic material of the subject, which is reasonable enough. Then they kick it into Dramatic Storytelling and decide they don't just need a sample, they need the body. The dead body.
There are multiple reasons why this is a Bad Plan. Death is usually permanent, so if there was a screw up and they need the subject alive, too bad; corpses aren't very talkative. It tends to give focus to the subject or their friends and make them fight you much harder than if you just wanted some blood samples. It's terrible PR, since the general public frowns on experimentation via murder. It's much better if the subject is alive so they can avoid all of those pitfalls and generate more genetic material or blood. However, captivity isn't as dramatic as impending death.
- Casshern of Casshern Sins is immune to the Ruin that affects every other robot, and consequently there is a rumor that eating him will grant eternal life. This is hinted at being true since Friender, a robot dog that took a chunk out of him, shows no sign of Ruin years later, but no one asks to slice off a chunk of his regenerating body, they just charge into battle and die.
- In Stitch! The Movie, Hamsterveil tries to make a clone army of Stitch, which would involve killing the original.
- Averted in Pirates of the Caribbean: The Curse of the Black Pearl initially, then played straight.
Elizabeth: That's it?
- The pirates avert the trope with Elizabeth. They let her believe she'll be sacrificed to free them, but all they need is a few drops of her blood on the gold.
Barbossa: Waste not.
- When it becomes William's turn, they are by this point rather frustrated and decide to spill it all just to make sure. And provide greater dramatic emphasis for the effort to rescue him, of course.
- MNU in District 9 does some tests on Wikus to confirm his ability to use alien weaponry, then decides to kill him to examine his genetic material.
- In Man of Steel, the Kryptonian survivors need an object called the Codex, which contains the DNA plans for reconstructing their species. One of General Zod's scientists discovers that the Codex has been fused with Superman. All it'd take to retrieve it is a cellular extraction, but Zod has vengeance on the mind, and asks if Superman has to be alive for them to retrieve it. Turns out he doesn't.
- Inverted and invoked by Nyah in Mission: Impossible II, who injects herself with the last sample of a Synthetic Plague that the villains need for a Poison-and-Cure Gambit to stop them from killing her. All of the parties involved simply assume that it would be impossible to retrieve a sample of the virus from a freshly-dead corpse.
- X-Men: Days of Future Past: After studying Mystique's DNA, Bolivar Trask decides he wants more samples in order to create new kind of technology with Adaptive Abilities. Among the samples he needs are brain tissue, spinal fluid, and bone marrow, implying that, if he got the chance, he would pretty much pick Mystique apart piece by piece.
- In Halo: First Strike, Sergeant Johnson turns out to be immune to the Flood. Halsey gives John the choice of giving the Office of the Naval Intelligence one of two data crystals. One directly mentions Johnson by name and and outlines how his nerve condition makes him immune, the other has the same information but omits his name and DNA. She warns John that giving ONI the former will likely cause them to dissect Johnson in hopes of figuring out the cure. John struggles with which is better to give, and chooses the latter.
- There was an Animorphs book with a race of inbred mutant fish people who wanted to kill the Animorphs with an elaborate machine to get their DNA to increase their gene pool. They aren't interested when it's pointed out that there's easier ways to get a DNA sample.
- Averted in the Vorkosigan Saga story, "Labyrinth", where Miles is sent on a mission to retrieve a sample stored in the muscle of a genetically engineered super-soldier. He's given a lethal injection for the super-soldier, but it's unrelated to fetching the sample. Rather, the scientist believes that Miles won't help rescue the said super-soldier and death is more merciful than what she faces.
- In Clan of the Cave Bear medicine woman Iza teaches Ayla about the healing properties of various plants. But Ayla pulls the whole plant, killing it, so Iza has to teach her not to take the whole plant so that it will continue to grow and provide medicine for the future.
- Addressed in a Metal Hurlant Chronicles episode, where the subject willingly gives his blood and life to save his dying princess, specifying that all the blood is needed. However, it's still played straight since no effort is made to capture the blood and we last see her lapping at his cut throat with a large pool of blood on the ground.
- In the Star Trek: Deep Space Nine episode "Who Mourns for Morn?" a group of thieves show up on the station to get at Morn's horde of Gold Pressed Latinum from a heist they had all participated in years earlier, and which Morn has willed to Quark. The delivery is coded to Quark's thumbprint, and one of the thieves threatens to kill Quark and take his thumb if he doesn't cooperate. One of the other thieves points out that it'd be more than a little suspicious if they showed up with a bloody severed thumb to get at the delivery.
- In an episode of M*A*S*H Margaret thinks she might be pregnant. She asks Hawkeye to do a Rabbit Test on her (where they inject a rabbit with a urine sample from a possible mother and then dissect the rabbit to examine its ovaries). The only female rabbit available is Radar's, who refuses to allow them to kill his pet, but he will let them do the test if they promise that the rabbit won't be killed. So the doctors do an ovarectomy on Radar's rabbit.
- There's an episode of Diagnosis: Murder in which the killer turns out to be a doctor who specializes in studying genetics, and his intended victim has a unique trait in her genes that he's convinced will further his research.
- Not quite killing but still excessive, in the Farscape episode "DNA Mad Scientist" the scientist wants a very large sample of Pilot's DNA, one of his arms (it regenerates). Of course earlier he'd claimed that he needed to take genetic samples from the crew's eyes in order to determine the locations of their homeworlds, and when Crichton asked why he couldn't just use some skin he stated that would be the case if he working at the level of mere DNA.
- Sliders had Colonel Rickman who killed for brain tissue to combat a brain fungus he picked up in one of his alternate universes' wars.
- In the Doctor Who episode "Deep Breath", both a chatty human passerby and a tyrannosaurus are killed by clockwork robots that have a use for some of their eye tissues. The man, at least, could surely have survived without his eyes.
- In Dragon Ball Online, it was Miira's goal to travel through time and do this to Goku, the strongest warrior in history, in order to make himself more powerful.
- In The Last of Us, the Fireflies want to kill and dissect Ellie to see if her immunity to the cordyceps fungus can be adapted into a cure. Justified in that the fungus grows on the brain: it'd be virtually impossible to nonlethally get what they needed from Ellie.
- Accidental Centaurs went on hiatus with a cliffhanger about whether Lenny is going to kill Stavvros to get a DNA sample.
- In a Treehouse of Horror episode of The Simpsons it's discovered that Bart is immune to the Zombie Plague, so a bunch of other (non-zombie) people plan to eat him in order to become immune. Eventually he just swims in the soup they all eat.
Leela: Professor! Can't you examine my brain without removing it?Farnsworth: Yes, easily.
- "A Clone of My Own". To sneak into the Near-Death Star to rescue the Professor, the crew need a DNA sample from him. Fortunately, they can get one from the Professor's clone, Cubert. Rather than one cell, however, Bender takes out a liter's worth of Cubert's blood, telling the guard to "keep the change".
- "Cold Warriors". To stave off a cold epidemic (the common cold was eradicated centuries ago and people no longer had immunity), the Professor needs to get a sample from Fry... by grinding him up into a slurry. Fortunately, Fry remembers that he had a cold sample sent to space when he was a kid, and they decide to spare Fry and get that sample instead, much to the Professor's disappointment.
- "Rebirth". The Professor uses stem cells to revive the crew. When Fry points out how controversial stem cells were in his time, Farnsworth explains that he's using adult stem cells... harvested from adults whom he had killed for their stem cells.
- In "The Sting", Professor Farnsworth prepares to test Leela's brain to see if she's sane... by putting her in a guillotine.