The whole point of a villain is to oppose a hero. They could turn out to be sympathetic, misunderstood, or tragic—but there is usually at least some attempt to antagonize the main characters. However, there are plenty of times when anime villains accidentally help the heroes along their journey.
Sometimes, it's taking care of the other villains, and other times, it's helping the heroes fully realize their power. More often than not, these villains rarely get to survive after giving the heroes an accidental helping hand, especially if they expose a way to take them down. Though in all fairness, heroes accidentally helping the villains, in turn, isn't exactly unheard of, either.
10 Sailor Moon: The Witches 5 Defeated Each Other
In the anime adaptation of Sailor Moon, the Sailor Senshi didn't need to do anything to get rid of the Witches 5 since they tended to either defeat each other. Mimete removed Eudial's car brakes, Tellu trapped Mimete in virtual reality, Tellu was defeated by her own plants, and Viluy was defeat by her nanobots. The final witch, Cyprine, somehow manages both as she and her alter-ego, Ptilol, defeat each other. Even Kaolinite ended up digging her own grave by abusing Hotaru, whose alter-ego Mistress Nine thought she used up her usefulness.
This didn't happen in the manga, but it's generally assumed the anime didn't want the girls defeating the more humanoid villains, either for censorship issues or to make them more of a threat. As some fans noted, this emphasizes the importance of teamwork and friendship: the villains might have won if they didn't keep betraying each other.
9 Detective Conan: Gin Accidentally Protected Shiho Miyano's Real Identity
Pisco was a major financier for the Black Organization. However, he was killed by Gin after he was caught on camera taking the life of a politician.
This proved beneficial to Conan since Pisco knew that Shiho Miyano—also known as Ai Haibara—was alive and had shrunk herself. He could have revealed that information to the organization if he was alive, but he ended up taking that information to his grave.
8 Dragon Ball Z: Frieza Just Seems To Bring Out The Best In Goku
After Frieza had proven himself powerful enough to use a Spirit Bomb and left Piccolo mortally wounded, he decides to be petty and attack Krillin. This helped Goku become enraged enough to enter his Super Saiyan form, allowing him to take down Frieza.
In general, Frieza's actions have been somewhat beneficial to bringing out the best in Goku. His destruction of the Saiyans caused Goku to be sent to Earth, giving it such a powerful hero to protect it and kept him from being raised with the sadistic, ruthless influence of the other Saiyans.
7 DokiDoki! Precure: All Those Monsters-Of-The-Day Wasted Valuable Evil Energy For The Invasion
Towards the end of the series, the villains hope to invade Earth and take advantage of human's inborn selfishness to accomplish this, hoping the townspeople will riot and betray each other, essentially doing the work for them.
Unfortunately for them, the villains have already created so many monsters-of-the-day out of the selfishness from the people in town that the Cures have already purified them, meaning they end up trying to help each other against the invasion. To emphasize this, the villains sometimes unleashed more than one monster for some episodes.
6 Fullmetal Alchemist: Sloth Helped Fix Major Armstrong's Arm
During a fight against the Homunculus Sloth, Major Armstrong hurts his arm. Sloth took advantage of this to continued the fight. Doing the noble thing and leaving the wounded opponent alone would have been pragmatic for Sloth.
During the fight, Sloth hits Major Armstrong in just the right place and pops his shoulder back into place. In the end, Sloth is defeated. As the personification of laziness, Sloth is happy to die since this means he won't have to work anymore.
5 Inuyasha: A Moth Yokai Helped Inuyasha Get His Priorities Straight
Gatenmaru was a moth yokai who posed as the human leader of a group of bandits. He once trapped Inuyasha and Miroku and forced them to watch to devour women he stole from a village he destroyed with his minions. Eventually, he went after Kagome.
This caused Inuyasha to give into his yokai blood and enter his full power, allowing him to kill Gatenmaru, whose death was almost instantaneous in the manga, and his minions. Not only did Gatenmaru effectively seal his own fate, but the incident also had a lasting impact on Inuyasha. When Inuyasha recovered his sanity, the experience made him give up on his desire to become a full yokai.
4 Inuyasha: Taigokumaru Really Shouldn't Have Killed His Son Or Boasted About It
Taigokumaru, a bat yokai, had found a way to become practically invulnerable. His granddaughter Shiori was able to create a protective barrier that even Inuyasha seemed powerless to break through. However, the one catch is that Shiori needed to keep the barrier up. When Taigokumaru began to boast about how he killed his own son Tsukuyomaru, who is naturally Shiori's father, she expels him from the barrier, allowing him to be destroyed by Inuyasha.
Shiori even offered Inuyasha the chance to destroy the crystal that gave the bat yokai their barrier powers. Taigokumaru tried to get revenge as a spirit but was ultimately defeated by Inuyasha with the help of the spirit of Tsukuyomaru.
3 My Hero Academia: The One For All Quirk Was Created To Help Others
All For One, the creator and mastermind behind the League of Villains, turns out to have accidentally created the One for all quirk that All Might and Deku, in addition to their other predecessors, have used.
All for One gave his brother a quirk without realizing he had a transferring quirk the whole time, which fused the quirks to create the One for All quirk that has been a problem for All for One ever since. One arguable advantage the Quirk does give All for One is that it gradually ages the user to death. But even this eventually gets a workaround: users without a natural Quirk are immune to this catch.
2 Tokyo Mew Mew: The Main Villain Created Helpful Alter-Egos
Deep Blue is the main antagonist of the series, and he wishes to reclaim the Earth. However, his alter-egos would prove less villainous. Masaya, who initially did hate humanity and only behaved otherwise as an act, ended up falling in love with the heroine Ichigo and even sacrificed himself to save her.
In fact, Masaya's love for Ichigo became so powerful he developed another alter-ego to protect her, the Blue Knight, neither of which seemed initially aware of being the other.
1 Yu Yu Hakusho: Killing The Main Character Backfired
Shinobu Sensui killing Yusuke wouldn't be the end of Yusuke. After all, he's come back from death before. And this time, it turns out that he's a Mazoku, a descendant of a yokai clan, and he awakens from death to his full yokai powers, likely thanks in part to having built up his latent supernatural powers as a Spirit Detective. Yusuke had come back more powerful than before.
After this, Raizen took over Yusuke's body and delivered a fatal blow against Sensui. Ironically, Sensui had wanted to die at the hands of a yokai anyway.