Avatar: The Last Airbender Revised firmly established itself as possessing Anyone Can Die from the beginning, with the passing of Sai's friend Xin Fu, Sakodi's friend Enri, and numerous Kyoshi warriors, among others, in the first book alone. By the end, Ain, Hakoda, Teo, Azul and Aizo are dead, and Zotu is Faking the Dead.
The author of Bad Future Crusaders, although promising she won't just kill characters left and right for the heck of it, has made a point of claiming that no character is safe since it makes for a far more interesting read. Seemingly to hammer this point home, this was said right after Twist who was built up as a major character, was killed very unceremoniously by Merrilay.
The Blood Bond, Blood Omen Series of fanfics for Kim Possible kills off familiar characters with reckless abandon. Among the casualties are Dr. Director, Monkey Fist, Gemini, Sensei, Frederick, Hirotaka and Adrena Lynn's face.
Crossover series Both Syllables has so far killed off three fairly major characters. Two of these were solely because the author is irritated by the amount of Disney Deaths in the source material.
Children of an Elder God: The only main characters survived the war were Shinji, Asuka, Touji, Hikari and Misato. Everyone else died... but Ritsuko and Maya, although it is revealed in the last scene.
Child of the Storm is a milder example of this, since while it killed off Sif and Harry Dresden after pulling a Tonight, Someone Dies, both were pulled back from the brink with the 'I never said it would stick'. Then, suddenly, major character Baron Von Strucker is killed off in chapter 51, and in chapter 72, Arthur Weasley and Luna Lovegood are both killed off. In the latter case it's a particular kick in the teeth, as their lives were getting better - Arthur's set to get a high paying job at Stark Industries and Luna's been taken under Harry's protective wing. Instead, Arthur gets a Neck SnapMercy Kill from the Winter Soldier and Luna chokes on her own blood thanks to a stray bullet from HYDRA's assault on Hogwarts - though she winds up becoming the new Delirium.
Also killed off on-screen are James Bond though he's later resurrected by Wisdom as a sort of techno-zombie; a number of unnamed homeless people; a young family who Gravemoss decided to test the veidrdraugar on; King T'Chaka of Wakanda; most senior members of the Ministry of Magic, including Rufus Scrimgeour and possibly Madame Bones; the Griffin; the entirety of HYDRA's Hogwarts assault team (excepting Daken); and Mulciber.
Also killed off-screen are most of MI6 and MI13.
A significant proportion of the Winter Guard meet a sticky end at the hands of Magneto in the sequel, Ghosts of the Past, and the Beast a.k.a. the Blob a.k.a. Dudley Dursley ends up meeting his at the hands of Dracula, who turns him into a vampire, and then Harry, who dismembers him.
The fanfic series A Dance of Shadow and Light by Ocadioan plays this trope to a T after the third story, where the author gives no thought to killing off characters(both minor and major) left and right, like the time when a sister-in-law and mother to a main character's niece dies off screen and for no apparent reason or purpose, as opposed to the first story where only a reluctant Dragon dies near the end, and the second story where only the male main character's established love interest was killed by his own hands, though that was more a case of I Did What I Had to Do.
Dangerverse. Similar to the original series, though by the end of book 4 it nearly rivals the whole series. Quirrell, Lockhart, Siss, Andromeda, Cedric, Marcus, Umbridge, Frank Longbottom, Penelope Clearwater, a little girl not even in Hogwarts yet, Hagrid, and the last book isn't done yet.
Downfall takes a sharp turn from the original source, where deaths are unheard of; or at least, don't stick. Though several characters start out alive and kicking, when in canon they were dead and buried long before the series started, the second major engagement in Unohana's war sees Rukia dead.
In Dumbledore's Army and the Year of Darkness, in the Battle of Hogwarts, the brutal deaths of Dennis, Michael, Cho, Parvati and too many others come as shocks. Continued with alarming enthusiasm in the sequels. Basically, anyone who did not get an explicit name check as being alive and well in Rowling's epilogue can be presumed to be dead by the time the trilogy has run its course. And even that is no guarantee, because "Epilogue? What Epilogue?"
In the pony fanfic Envy and Arrogance, this is expressed in three stages. First, most of the population of Equestria dies off-screen, including several minor characters. Then, the stakes are taken higher with a Tear Jerker death of Apple Bloom. More deaths follow. Finally, we learn that the original Elements of Harmony managed to save the world despite losing several members (either to death or mental breakdown) - that means even the mane six's Plot Armor can no longer be depended upon.
In Future Tense, it starts with three characters dying within the first few paragraphs and only goes downhill from there.
With the Gensokyo 20XX series being a series of primarily Dark Fics and Death Fics, it comes with this trope and even immortals and children are no exception, seeing as Eirin, being one of the aforementioned, was Driven to Suicide and succeeded and the fact that the series has a rather high body count.
Happy Families Are All Alike features the Third Hokage dying in the very first chapter. Later, we find out that Jiraiya and Tsunade were killed off by Orochimaru, who has also successfully possessed Sasuke.
The Ace Attorney fic Law Plus Chaos has many characters killed off, most of them in VERY brutal ways, such as one person being cooked into pies.
Played with in Series Five of The Lion King Adventures. Tojo and Tama die in grisly ways, but unlike some others who are brought back their deaths remain permanent.
The Fanfic Pedestal. Human or Pokemon, Original or Canon Character. Anyone could be next.
The Sun Soul, a Pokémon fic, fits this trope in every way possible. The author spares no pains in developing a character, making the audience identify with them, then ripping the character away in a brutal, often jarring, manner.
Thousand Shinji: For the end of the fic, the only canon characters left alive are Shinji, Asuka, Rei, Misato, Toji, Kensuke and Hikari. Everyone else is dead. The body count is even greater than it looks, since Asuka, Misato and Hikari got murdered and were brought back from the dead.
The Command & Conquer fan-novelization Tiberium Wars makes it a point to often, suddenly, and repeatedly kill off well-developed characters. Some characters get less than a scene to their names before they get slaughtered, and you've really got no idea if a character is going to survive until they bite it.
Most of Hans Von Hozel's fics end on a melancholy note. For example: He kills all of the Ninja Turtles and Splinter in his story Ninja Turtles and in his Doctor Who fic, the Doctor gets eaten by raptors.
Zombies, a videogame crossover where a Zombie Apocalypse is corrupting the world of video games. Anyone who gets bitten or infected will become a zombie, will gain superhuman strength, and a relentless urge to kill and devour the living. And the best place to do this is in the idyllic Mushroom Kingdom where even turtles can pose a significant threat. Almost anyone who might seem important is on the author's hitlist.
Played with in Demigod Power: Character deaths have been few and far between thus far, but this trope remains in effect because there is nothing stopping the writers from killing off any of their characters. Judging by the last two lines in the next Great Prophecy, this trope is going to be played straight HARD.
A Brighter Dark: Being a retelling of a Fire Emblem game, this might have been expected. Except this trope extends even to characters that were protected by plot in the original game. Disney Death is nonexistent, the protagonists are absolutely devoid of anything resembling a Thou Shalt Not Kill rule, and given the nature of the conflict, there's hardly even a 'good' side that's destined to win in the end.
The Warhammer 40,000 fanfic Secret War a lot of main characters killed off it gets to the point two out of three of the main characters who are introduced right at the start, wind up dead and one of them is Attelus Kaltos, the character whose perspective it's from.
Aside from the campaign's...interesting character creation session, the All Guardsmen Party has had at least three different deaths, not counting other team members they've been assigned to.
Crossover fanfic, Game of Mass Destruction, has a large cast of characters - many of them get killed off.
The RWBY story In the Kingdom's Service kills of a shocking number of it's main characters. Over the course of the story, Velvet, Cardin, Oobleck, Ciel, Coco, Mercury, Adam, Winter, and Cinder are all killed. The first three were all members of the Vale Secret Service and Jaune's mentors, and Coco, Velvet, and Ciel all die offscreen.
Arrow: Rebirth: Basically, if you aren't the counterpart to a major comics hero (or a decomposite of one, in the case of Laurel and Sara), you're free game. The author hasn't even made it to the equivalent of the mid-season finale and already he's killed off at least four major characters. At one point, he jokes that he's the George R. R. Martin of the Arrow fan community.