If one looks hard enough, they can find that some villains possess fertile soil that allows the seeds of goodness to grow. If someone wipes away the crud, the gank, and all things bad, and carefully nurtures the seeds of good intentions, it can lay the foundation for saplings of courage, kindness, and heroism to grow in their hearts. Though, other villains are just bad to the bone. The barren soil only allowing wickedness, villainy, and bad intentions to flourish. Evil just having too much of a hold on their soul.
Examples of each exist in Yu Yu Hakusho. Several demons (and some half-demons) go through the process and end up one way or the other.
10 Stayed A Villain: Gōki
Gōki was part of Yusuke Urameshi's first case as Spirit Detective and his first real fight with a demon. It may be hard to tell who he is in his human form, but the soul sucker possesses a demon form that makes him look like the Oni of Japanese lore. Complete with rock hard skin that no normal human could penetrate.
Best described as a two-bit crook, thankfully he wasn't allowed to become anything more thanks to a well placed Spirit Gun through the mouth. Gōki's bad intentions, appetite, and his tendency to use children's souls to satisfy both going down with him.
9 Became A Hero: Suzuki
Like Gōki, it might be hard to tell who this person is without their (clown) make-up. Suzuki is the leader of Team Uratogi, a team mostly compromised of characters that serve as perversions of classic Japanese fairy tales.
When the Team Urameshi first meets Suzuki, he is nothing more than a narcissistic (literal) clown with his head so far up his butt that he could barely see what a threat Genkai was. After a well-deserved beat down by her hands, Suzuki manages to even himself out and become a bona fide ally of Yusuke and friends. Offering them assistance via magical items for their fight with Team Toguro and later assisting Yusuke during the Three Kings Saga.
8 Stayed A Villain: Randō
Another villain whose reign of terror was cut short thanks to Yusuke Urameshi. Basically a head hunter for new abilities, his modus operandi was to learn a new ability and then promptly kill the person who taught him. Something he completed ninety-nine times before running into the Spirit Detective.
What's worse is that Randō is a complete sadist. One of his main motivations for learning so many techniques was so that he could use them to find new ways to torture his opponents. Randō eventually takes those bad intentions with him to spirit jail after a shrinking technique backfires on him and Yusuke elbow drops him to submission.
7 Became A Hero: Raizen
Once upon a time, Raizen was the biggest and worst demon this side of the Human and Demon Worlds. With power incomparable to any demons that currently exists, and twice as vicious with an appetite for humans. So what exactly made the deadliest demon of all time have a change of heart... well... nothing more than sweet, sweet love.
After being nursed back to health and then falling in love with the human woman who did so, Raizen immediately took humans out of his diet. Raizen preferred to die than ever eat another human, a decision that eventually did kill him. He later helps his great-grandson defeat Sensui during the Chapter Black Saga and then advised Yusuke to side with Mukuro in an attempt to protect both the Human and Spirts worlds after his death.
6 Stayed A Villain: Roto
Roto has the distinguished honor of being the most despicable demon that Team Urameshi faces in the Dark Tournament. With no real power to speak of, Roto uses Kurama's mother as a hostage to try and take the victory. Relishing the chance to try and make Kurama his punching bag before trying to kill him off.
Needless to say, Roto died as he lived, a cowardly villain. The low-class demon sniveling at Kurama's feet once he took the upper hand and begging for mercy. The Death Plant soon blooming inside of him and ending his life before he would ever get the chance to turn his life around. His death serving as a warning to not mess with Kurama's mommy.
5 Became A Hero: Kurama
Before the start of the series, Kurama was the Legendary Bandit Yoko Kurama. A vicious thief that would even backstab the future king of the Demon World, Yomi, and was partly responsible for blinding him. The selfish demon only really living for himself and the thrill of a good heist.
That is until Kurama meets his human mother. After implanting his soul into the body of his mother's unborn child, he grew to care for the woman who was willing to put her life on the line for him. That love later extending to his Yusuke and his friends after becoming indebted to the Spirit Detective.
4 Stayed A Villain: Karasu
This mad bomber is a man who must destroy the things he loves. The victim of his twisted affection being Kurama during their fight in the Dark Tournament.
Karasu won't just kill them downright. He would rather physically and mentally torture his targets to savor what he believes to be the intimate relationship between a killer and their victim, slowly taking them apart with non-lethal explosions until they fall to dust in his hands. Kurama eventually sending that sick and twisted personality to the grave as Karasu's tendency to play with his victims backfires and gives Kurama a chance to strike back.
3 Became A Hero: Hiei
Hiei is rough around the edges but he still has a soft spot for his half-sister Yukina. That soft spot serving as a catalyst that ultimately gets Hiei to work with Yusuke beyond just serving his parole. Eventually forming true friendships with all of them.
In the end, Hiei goes from being a one-note villain at the beginning of the series to being a valuable member of Team Urameshi and then being tasked with escorting lost humans out of the demon world. Though he never truly loses his killer instinct, it is still one giant step above the villain he was at the start.
2 Stayed A Villain: Elder Toguro
Elder Toguro is nothing but body horror in its most vicious and dastardly form. Able to shape his body in whatever way his devious little mind could think of, he uses his abilities to slice and dice his opponents to ribbons. All the while taking immense pleasure in torturing them.
Unlike his bother, Elder Toguro doesn't have an ounce of honor in his body. More than happy to cheat and help his brother do the same, it was that later offer that proved to be the last straw for Younger Toguro and he punched Elder Toguro into oblivion. Though, having survived that, Kurama put the finishing touches on him by later trapping him in the Sinning Tree. A demonic tree that will provide him with constant pain and hallucinations until his life force is drained. Of course, he could find redemption while suffering through the tree's delusions, but it is doubtful.
1 Became A Hero: Yusuke Urameshi
It may be easy to forget but Yusuke was a downright punk at the beginning of the series. Left with a missing father and a mess of a mother, he lashed out at everything and everyone possible. Which included some people who might have deserved it, like the teachers at his school and other punks, and others who didn't, like his friend Keiko.
It was only through his death and resurrection that Yusuke gained any semblance of heroism. Eventually, Yusuke uses his dumb muscle and bad attitude to protect those around them as a Spirit Detective. The little punk eventually becomes a defender of the realm and helps to foster peace between the Human, Spirit, and Demon Worlds by the end of the series. Pretty good for a punk kid.