10 Times An Anime Hero Let A Villain Go (Only To Regret It Later)
David Zimmerman
Bleach Nnoitra vs Nelliel, AOT reiner vs scouts
Bleach Nnoitra vs Nelliel, AOT reiner vs scouts
Frequently, the concept of mercy is presented favorably in anime narratives. A hero's willingness to spare a villain either does not come back to haunt them or provides a window for future redemption arcs.
However, there are just as many instances where the most logical outcome runs its course. Emboldened by their escape from death, the villain returns in the future to make the hero regret showing mercy on them. By identifying examples of the trope, we can better comprehend how surprisingly prevalent it is across many different stories and universes.

10 Jean's Decision To Spare Reiner Enabled His Escape (Attack On Titan)

reiner thunder spears attack on titan
reiner thunder spears attack on titan
Jean and the other Scouts were exceptionally merciful to Reiner despite his role in Shiganshina's downfall. They had two separate chances to kill him during their second battle, both of which would have ripped the Armored Titan from Marley's hands.
Because they hesitated, Pieck helped Reiner retreat from the island in a daring rescue. From there, he conveyed valuable information to his superiors and helped to prepare for a full-scale invasion of Paradis. While Reiner would be an ally during the Rumbling, his Armored Titan still claimed the lives of many soldiers.

9 Yugi Didn't Finish Off Kaiba When He Had The Chance (Yu-Gi-Oh!)

After Kaiba stole the Blue-Eyes White Dragon, he and Yugi had a duel in order to decide its fate. Although the deck and odds were literally stacked against him, the Pharaoh ultimately emerged as the victor.
Since the Penalty Game he placed on Kaiba was not severe enough, however, the card mogul was able to return to Yugi's life, kidnap his grandfather, and force his friends through a series of death-defying games. If Yami had subjected his enemy to a harsher punishment, Kaiba may not have been as prepared for revenge.

8 Since Luffy Didn't Finish Off Gecko Moria, The Villain Reappeared At Marineford (One Piece)

Gecko Moria was a maniacal warlord and the owner of the Thriller Bark. He kidnapped Luffy and his friends shortly after the escape from Enies Lobby and subjected them to a world of horrors.
After narrowly triumphing over the villain and his monstrous giant, Luffy strangely decided to spare his life. Gecko Moria would take advantage of Luffy's mercy by showing up at Marineford and skewering Oars Jr. through the chest. Because of his actions, it was significantly harder to rescue Ace from the scaffold.

7 Nnoitra Betrayed Nelliel's Trust (Bleach)

Espada five vs espada 3
Espada five vs espada 3
Nnoitra was insecure about Nelliel's higher ranking due to his hubris and misogyny. He failed to defeat her in their initial duel, prompting him to consult Szayelaporro Granz for help.
In their next encounter, his newfound technological advantage would see Nelliel defeated. She was cast into the dunes of Las Noches alongside her Fracciones, having been so traumatized from the event that she physically regressed into the body of a young girl. If Nelliel had killed Nnoitra after their first fight, she would have been able to avoid catastrophe.

6 Elizabeth Spared Derieri & Monspeet, Only For Them To Turn On Meliodas Later (Seven Deadly Sins)

Elizabeth saves monspeet and derieri
Elizabeth saves monspeet and derieri
During the war between the Supreme Deity and Demon King, Ludociel was in an excellent position to finish off Derieri and Monspeet. However, Meliodas forced him to stay back while Elizabeth reverted the demons back to their original and safer forms.
Despite this mercy, both lovers would assist Zeldris in attacking and defeating Meliodas upon the Ten Commandments' return to Britannia. They helped Estarossa kill the captain and bring him one step closer back into the demons' ranks.

5 L Was Forced To Release Light Since There Wasn't Enough Evidence (Death Note)

Death Note Light and L split screen saying "I am justice"
Death Note Light and L split screen saying "I am justice"
Having suspected Light for a long time, L arranged a simple test to incriminate him. If the Death Note killings stopped after his imprisonment, Light's guilt would strongly be implicated.
However, the murderer had anticipated such a crafty move and handed off his notebook to the Yotsuba group in advance. The wealthy businessmen agreed to conduct Kira's scheduled killings under the condition that they could also murder powerful rivaling company executives. Although a lack of evidence forced L to let Light go, he would live with the consequences for doing so later.

4 Snipe Missed His Opportunity To Kill Shigaraki Since He Wanted To Take Him Alive (My Hero Academia)

Snipe shoots Shigaraki down
Snipe shoots Shigaraki down
After the first invasion of U.A. ended in failure, Shigaraki began to prepare his escape. However, professional hero Snipe had already arrived and landed dozens of non-lethal shots against the villain's arms and legs.
Since he deliberately didn't aim for vital organs, Shigaraki was able to escape from the school alive. As a result, the League of Villains would be able to continue on for years to come, even after All For One's capture.

3 Steely Dan Almost Hurt An Innocent Girl Because Jotaro Was Going To Spare Him (JoJo's Bizarre Adventure)

After Joseph Joestar was freed from Steely Dan's "Lovers" Stand, Jotaro turned around and prepared to leave. As sincere as Steely Dan phrased his apologies, he almost immediately betrayed the hero by attempting to take a nearby girl hostage.
However, Kakyoin's Hierophant Green had already wrapped itself around the microscopic Stand's leg. As a result, he was able to extract it safely from its latest victim before any harm could be done. Realizing the mistake of his leniency, Jotaro promptly pummeled Dan into the ground.

2 Hashirama's Decision To Spare Madara Caused Massive Problems (Naruto)

Dissatisfied with the treaty between the Senju and Uchiha clans, Madara attacked Hashirama at night with the aid of an enslaved Kurama. Despite being the second time he had spared Madara, the First Hokage nonetheless allowed Madara to slink away in disgrace after his latest defeat.
His mercy had severe consequences for future generations. Having stolen Hashirama cells, Madara was able to create White Zetsus that he used as his eyes and ears on the surface. Further, he turned Obito in a weapon against the entire world during the Fourth Shinobi War.

1 Levi Immediately Regretted Sparing Zeke Since Pieck Saved Him (Attack On Titan)

After Levi defeated Zeke, he attempted to take him alive. Despite the dangers the man posed, his Beast Titan could have been a significant help to the Scouts if fed to one of the dying troops. The captain had a keen interest in reviving Erwin if possible.
Pieck's interference would cause Levi to regret his decision immediately. By lunging into the battle from a thick cloud of smoke, she rescued Zeke from Levi's clutches and ensured that her friend's shifting ability would never fall into Paradis' hands.