It is common for an anime villain to officially turn over a new leaf through some memorable act of heroism. This sets a precedent for the rest of their story arc and christens them a new person through their actions.
However, it has also been true that they would be inclined to help a hero in a single specific instance before returning to villainy, especially if their reason for lending a hand was self-interest. By analyzing examples, we can appreciate their uncharacteristic assistance and the conditions behind it.
10 Kabuto Only Joined Team 7 To Manipulate Them (Naruto)
Kabuto joined Team 7 during the Forest of Death as an ally. In greater numbers, the heroes stood a better chance of thriving in the exam, so they chose to have him join the group.
However, the medical ninja's ulterior motive would set the framework for the rest of the story. He sought to appraise his "comrades" and report what findings he made to Orochimaru. After the Sannin heard of Sasuke's whereabouts, he confronted him, and he was one of the causes for Sasuke's descent into villainy.
9 Wamuu Gave Caesar His Lip Ring As A Token Of Respect (JoJo's Bizarre Adventure)
Wamuu's lip ring was the antidote for what poison was slowly dissolving in Joseph Joestar's chest. After he prevailed over Caesar, he allowed the Hamon user to keep the cure as a sign of respect for their battle.
Despite this act of honor, Wamuu would remain loyal to Kars until his death. This was a miserable fate since his chivalry characterized him as a potential candidate for redemption.
8 Hisoka Gave Gon His Badge And Helped Defeat Razor (Hunter X Hunter)
Hisoka's relationship with Gon may be creepy, but he proved to be one of his most useful assets throughout his journey to find his father. The first instance of his help was through the Hunter Exam, where he willfully let Freecss keep his badge to continue with his trials.
He would assist him again in Greed Island, where they—alongside Killua—would collaborate to defeat game master Razor. However, Hisoka's contributions are only to carefully groom Gon as a formidable opponent to murder in the future.
7 Envy And Edward Worked Together To Escape Gluttony's Stomach (Fullmetal Alchemist)
Envy is a notably unlikely ally to Edward for his notorious hatred of humans. However, after Gluttony unwittingly swallowed both of them, they were forced to collaborate to escape the fake Gate of Truth that was the corpulent homunculus' stomach.
This was far more than Envy's self-preservation. He acknowledged that Edward was imperative to Father's plans and would have been useless if their master could not physically obtain him. After they were freed, the shapeshifter remained a villain until his suicide after the story.
6 Reiner Saved Everyone By Throwing A Titan From A Tower (Attack On Titan)
Determined to maintain his disguise, Reiner threw an invading titan from a tower at Utgard Castle's battle. This was at great expense to himself since he suffered grievous wounds on his arm.
However, his actions were to preserve his own identity rather than to save the other Scouts. Ironically, he would reveal himself anyway after the fight's conclusion, exposing Bertholdt in the process and forcibly abducting Eren Yeager.
5
Charon Saved An Entire City From Being Obliterated (Fire Force)
Charon was a White-Clad member whose ignition ability allowed him to reflect firepower with minimal damage to himself. After Ritsu cobbled together a massive monster using nearby infernals and one of the Eight Pillars, he realized that everyone was in danger.
By thrusting his body in front of the aberration's attack, he was able to absorb its potency and redirect it against the surface of the moon. Despite this flagrant act of heroism, it was only to ensure Shinra Kusakabe could fulfill his role in the Evangelist's grand objective.
4 Esdeath Saved Tatsumi From Giants (Akame Ga Kill!)
After Syura teleported Esdeath and Tatsumi to a remote island, they were quickly beset upon by massive monstrosities. Vigilant to protect her mate, the General leaped into action and cleaved them down.
Though it's uncertain whether or not Tatsumi could have defended himself with his Incursio armor, her actions guaranteed that he didn't have to, shortly after they returned to civilization, she would rejoin the Empire as Night Raid's most formidable enemy up until her death at the hand of Akame.
3 Stain Saved Midoriya From A Feral Nomu (My Hero Academia)
After Stain was captured, it seemed as if he lost all hope to fight. He was surrounded by heroes and bound in ropes, offering no resistance. However, once a feral nomu raced through the street and caught Deku in its talons, the vigilante shredded his bindings and brought the creature down.
This was due to the immense respect he felt for Midoriya as the future of hero society. Dolorously, it did not suggest that he might abandon his murderous inhibitions and attempt to reform himself under the pre-established system.
2 Sasuke Helped Naruto Fight Kaguya, Then Tried To Take Over The Shinobi World (Naruto)
After Sasuke joined the battle against Kaguya after the Fourth Shinobi War, it seemed as if Naruto had finally gotten through to his friend. However, once Otsutsuki was defeated, he revealed his own plans to dominate the entire planet.
This forced Naruto to fight him for a final time in the Valley of the End only after the avenger's second defeat was his thirst for evil permanently quelled.
1 Grimmjow Broke Out Orihime To Heal Ichigo (Bleach)
Grimmjow broke Orihime out of her cell to use her healing powers on Ichigo after he sustained grievous wounds. Though this was a tremendous boon to the shinigami's efforts in Hueco Mundo, it was not out of the sixth Espada's generosity.
Immediately after he was restored, Grimmjow expected a fair fight to settle his grudge with the hero. He was so hellbent on an even match that he would turn against Ulquiorra, banishing him so that their impending battle would not be rudely interrupted. Nonetheless, he attempted to murder Ichigo and reaffirm himself as the superior warrior.