While Goku, Vegeta, and the rest of the Z Fighters have faced their fair share of threatening villains, none were as initially frightening as Piccolo–a character who is now one of the greatest heroes in the Dragon Ball universe, but who was originally depicted as a literal demon.
Piccolo is a character with a pretty damning past, though despite his previously villainous ways, he managed to become a hero who would be integral to the many of the Z Fighters’ successes in battle. Piccolo’s first attempt at heroism came after the evil Saiyan Raditz first arrived on Earth and Piccolo teamed up with his arch enemy Goku to defeat him. Unfortunately, that fight resulted in Goku’s death along with the death of Raditz, which was something Piccolo thought he wanted, but became an event he would grow to regret. To make up for killing Goku, Piccolo decided to take Goku’s son Gohan under his wing and train the boy to become a powerful warrior. While Piccolo was still holding onto a shred of his former villainy during his training with Gohan, the half-Saiyan child managed to warm the Namekian’s cold heart and Piccolo eventually grew to legitimately care for Gohan, sparking his full metamorphosis into a hero.
Dragon Ball chapter 135 by Akira Toriyama introduced fans to the originally villainous Piccolo who was a far cry from the Z Fighter he would eventually become. In this chapter, King Piccolo was revealed to be a centuries-old villain who was known as the Demon King or Devil Emperor. More than a hundred years before the events of this chapter, King Piccolo rained terror down upon the world until he was stopped by one of the greatest martial artists of that time by way of the Evil Containment Wave–a technique that mystically traps beings in a vessel forever, unless the seal on the containment is broken by an outside force. When King Piccolo was eventually awakened he sought to pick up right where he left off and take over the world just as he tried to do so many years before.
Eventually in this saga, Goku was able to kill King Piccolo and the villain produced an egg that contained his reincarnated body, a being who would become the heroic Piccolo fans know and love today. Before his death, and even before he is revealed to be one of an alien species known as Namekians, King Piccolo had no prior knowledge of his heritage and legitimately thought that he was a demon king, which in turn made the world think that he was an actual demon. While he wasn’t spawned from the depths of hell, King Piccolo wasn’t too far off in his assumption about his origin. King Piccolo was what is known as a "demon-class" Namekian, which granted him the power to produce dragon-like offspring who would do his bidding without question. King Piccolo also had insane amounts of power, which he used for exclusively evil purposes.
After King Piccolo was reincarnated as Piccolo, the fighter was no longer a "demon-class" Namekian and instead was reborn as a "warrior-class" Namekian, making him much less evil during his second chance at life. While Piccolo's connection with Gohan helped him find redemption, he was originally one of the worst Namekians in existence–and before he was even aware of the fact that he was a Namekian, Piccolo and the rest of the Dragon Ball universe believed he was a literal demon.