Fan Works / Cycle Of Revenge

Guardians, Wizards, and Kung-Fu Fighters:
- Caleb wants revenge on Raythor for his father's death. Later, after Raythor has been framed and thrown into the Abyss of Shadows, Lothar seeks to avenge him.
- One of Tharquin's men points out that most acts of violence against humans and Galhots by Shapeshifters were retaliation for acts of violence against Shapeshifters, like when Tharquin occasionally lets one of his victims live to give people a reason to keep supporting him. Tharquin kills that man as a result.
Shadows over Meridian: Metalbeak killed a man who had killed numerous Mogriffs in the past, clearly enjoying it as he tortured them to death, even killing eggs and hatchlings. This man was Vera Bexley's beloved grandfather, and she has spent the ten years since that day plotting revenge against the Mogriffs in general, and Metalbeak in particular, for his death.

A Thing of Vikings: After Hiccup and his friends effectively curbstomp King Donnchadh mac Brian, Hiccup pointedly refuses to kill the king, citing this trope as one of his reasons. He doesn't want King Donnchadh to become a martyr, or to have an army under his banner seeking revenge against Berk. In defending his decision, Hiccup points out that the Cycle of Revenge is at the heart of the story of Ragnarok; Odin banished or tortured Loki's children out of fear, which prompted Loki to kill Baldr, which drove Odin to punish Loki, which would lead to Loki joining the jotunn against Asgard.

Checkmate Anlashok: The back and forth attrition between the Rebels and Capitol, and how the Capitol is prone to Moral Myopia about their murders while the Rebels view theirs as Dirty Business, become a major theme as things go on. One notable moment is when the Capitol causes a train derailment that kills the relatives of several victors, and the Rebellion responds by assassinating relatives (albeit ones involved in Capitol politics) of Caesar Flickerman after he tells Mags that what happened was her fault and shows an awareness of what really led to the train derailment.

In Friendship Is Magical Girls, one of these exists between the Soaring Lion Eagle and Emerald Flame Dragon clans. They were rivals for years, ultimately leading to a member of one killing the fiance of a member of the latter in a jealous rage. This caused the latter to fly into an Unstoppable Rage, wiping out most of the Soaring Lion Eagles and scattering the others around the world. Now, in the present day, Gilda — the last Soaring Lion Eagle — wants to wipe out the Emerald Flame Dragons in revenge.
RainbowDoubleDash's Lunaverse: Trixie and Galeb the zebrony's beef in "Title Match". She used to be friends with him, until she got his brother arrested for breaking the law. The rest of Galeb's family admitted his brother had it coming, but Galeb swore vengeance. He spends most of the story trying to get back at Trixie until she beats him, and tells him they could engage in one of these, but sooner or later someone will get seriously hurt. He agrees, and comes to a relative truce.
Starlight Over Detrot: Griffin culture was based on this, so badly that more griffins died during brief times of peace than during wars with foreign nations. Eventually, to prevent them from going extinct, the griffin ruler instituted a system for symbolically representing blood debts and trading these debts. By the present day, the griffins have built a complex economy on these blood debts.

Ffreire's NaruFox AU: Sakura and Shino discuss this while training together after the preliminary matches of the Chuunin Exam. Shino admits that after witnessing the way Sasuke broke the arms of his opponent, he was briefly afraid Sakura would follow her teammate's example during their match. Sakura notes that if she had done so, they both would've have extra reason to worry about Kiba and Naruto's match, since Kiba likely would've been eager to avenge his teammate by assaulting hers.
Space to Breathe: Sakura's parents are killed during the Konoha Crush, leaving her with a severe grudge against Orochimaru and Sunagakure. Eventually, she gets a chance to unleash her wrath upon the latter, using Water/Earth Release: Metamorphosis to transmute part of the land in the hidden village into water, flooding large sections of it. Upon surveying the aftermath and hearing the wails of survivors mourning their lost loved ones, she realizes the enormity of her actions and that it's exactly the sort of thing Orochimaru would have done were he able, causing her to throw up as the guilt sinks in.

A Push: Sansa goes through with shoving Joffrey off the ledge following her father's execution, killing him. In response, Cersei has Sansa executed by beheading. Then Robb Stark returns the favor by beheading Jamie Lannister.
In Chasing Dragons, Khal Pobo unites a massive Dothraki horde to wage war against the Kingdom of Myr in order to avenge the death of his own Khal, Zirquo, at the Battle of Narrow Run. In return, Robert swears revenge against the Dothraki for the depravities performed on his subjects during this conflict.

Sonic X: Dark Chaos: The Metarex War is basically a hugely-escalated version of this trope between Tsali and the Metarex. Also deconstructed as both sides are getting increasingly exhausted of the endless fighting. They finally break the cycle at the end thanks to Cosmo's intervention.

Flaihhsam s'Spahkh's Opening Narration discussed this. Romulans, particularly Romulan nobles, are prone to this due to the societal code of mnhei’sahe: a loss of face has to be rectified by word, deed, or blood, and can lead to cycles of revenge lasting for centuries. On the flip side, they also remember past favors and help with equal clarity, and so Romulan survivors revere Spock for stopping the Hobus event before it got any worse.

With This Ring: One of the alternate versions of the protagonist is in the Warhammer Fantasy universe, and has made a goal of actually settling the dwarfs' Book of Grudges. It's a big job, but his ability to massacre orcs and goblins has helped him to make great strides. The elves are surprised by the novel idea that their centuries of warfare with the dwarfs could actually be resolved fairly quickly with a proper accounting and payment of fines.

Cycle Of Revenge / Fan Works