Various villainous Frieza destroys Planet Vegeta in Dragon Ball, the last remaining Saiyans become saviors for the world and the whole universe, all due to Goku's contagious comion.
Some Dragon Ball characters that either resemble Goku or steal his body serve as Goku's dark mirrors, representing what could have happened if he weren't good at heart. If successful, these warriors could have turned Goku's iconic hairstyle into an ominous sign instead of a beacon of hope. But as fate would have it, every dark Goku parallel has been stopped, either by their lack of power, by self-sabotage, or by mere chance.
Young Goku
Saiyans used to send their low-level babies to distant planets trusting that their natural proclivity for conflict would allow them to conquer and expand their rule. Apparently, this method was effective, and it would have put Earth at Goku's feet if not for a convenient accident. When Goku was still a naturally unruly child under the late Grandpa Gohan's care, he fell down a ravine and hit himself in the head. Fortunately for the whole world, Goku's wicked personality disappeared and he forgot his mission to conquer the Earth.
Frieza's plan to eradicate all Saiyans didn't work when various warriors voluntarily or accidentally avoided their planet's destruction, but instead of rebuilding the brutal Saiyan race, Goku either eliminated or befriended the survivors. With their respective Saiyan-human sons, the now-reformed Goku and Vegeta not only defeated Frieza but also protected the world from powerful villains like the Androids, Cell, and Buu, and competed in the multiversal Tournament of Power, thus succeeding the little expectations Frieza and the Saiyan royals had about low-level Saiyan warriors.
Turles
In Dragon Ball Z: The Tree Of Might, one of Dragon Ball Z's many non-canon movies, the Saiyan Turles arrives on Earth to plant the Tree of Might and draw power from it. Turles, extremely similar to Goku in appearance and origin, only seeks to increase his power and destroy other alien populations in the process — exactly what Goku could have done had he retained his original personality. In fact, low-level Saiyan warriors tend to share certain physical characteristics like their hair and facial features, and they're usually expected to have the same goals if they don't spend too much time adapting to the cultures they're supposed to destroy.
Like Raditz and Nappa, Turles' pride and overconfidence led him to underestimate Goku and the Z-Warriors. He could have kept multiplying his power with the Tree of Might and only ed Goku when he was strong enough to dominate the fight. Instead, he went straight to Earth to taunt Goku and promptly got humiliated. An eventual Super Saiyan transformation by Turles was unlikely without Goku and Vegeta's level of training, but he could have had a better chance if he had a more humble personality.
Captain Ginyu
Captain Ginyu couldn't have been more different than Goku. He was a purple, black-horned alien from an unknown race with undying loyalty to Frieza and little power to speak of. However, Captain Ginyu had the ability to change bodies with anyone he wanted. He managed to switch bodies with Goku, but instead of using Goku's strength for his own benefit, Ginyu failed to understand how to use Goku's godlike power and soon tried to switch bodies with Vegeta, which also failed when Goku threw a frog between the two and trapped Ginyu inside the animal.
Captain Ginyu could have wreaked havoc as Goku. He only needed enough time to harness Goku's power and find his way off of Namek. As Goku, Ginyu could have conquered Earth or pressured almost anyone in the universe into obeying him. But Ginyu's greatest weakness was his short-sightedness. Even with one of Dragon Ball's most useful abilities, Ginyu ended up serving Frieza and changing bodies with a frog. The fact that he didn't even think of changing bodies with Frieza in the first place proves how Ginyu wouldn't have reached too far had he stolen Vegeta's body. Although he managed to survive long enough to change bodies with Tagoma in Dragon Ball Super's Golden Frieza arc, he still got himself killed after annoying Vegeta.
Goku Black
The most successful dark Goku parallel in Dragon Ball history has been Zamasu, who did what Captain Ginyu failed to do and permanently swapped bodies with Goku. Zamasu, an envious North Kai with a grudge towards mortals, traveled to the future and used the Super Dragon Balls to wish for Goku's body. This evil Goku might be the most tragic, as he went as far as killing Goku's family and destroying countless planets, claiming Future Bulma's life in the process.
As Goku Black, Zamasu had the chance to destroy many more worlds, but just like Turles, he got fixated on Goku and the Z-Warriors, which resulted in his defeat. Goku Black ed forces with a future Zamasu and fused with him, but their fusion went wrong when the Potara's effects ended, and the Z-Warriors were able to shatter the abomination they became. Goku Black's uncontrolled ambition led him to become a shapeless force of evil, and it took Future Zeno himself to erase him from existence. Needless to say, Goku Black is not only the most powerful version of an Evil Goku, but also one of the most powerful villains in Dragon Ball history.
Bardock
Goku's father, Bardock, used to be a cold-blooded warrior like most low-level Saiyans. Brutal like his comrades, Bardock may have been the last thing several alien races saw before their deaths. However, Bardock was also more intelligent than the regular Saiyan, as he deduced Frieza's plan to gather all Saiyans on Planet Vegeta before destroying it. While Bardock couldn't save his people, he proved himself a selfless hero at the very last moment. In every version of Bardock's heavily-retconned Dragon Ball story, he could have escaped by himself and made a new life in another planet, but he sent his son off-planet and tried to oppose his tyrannical boss with all the odds stacked against him. Besides, he was motivated by the deaths of his fellow soldiers, and he faced Frieza while wearing their blood on his forehead. Bardock may have been a simple foot soldier, but his sacrifice signaled the kind of selfless behavior that would characterize Dragon Ball's greatest hero, Goku.
