Piccolo is one of the Dragon Ball franchise's most important characters, and his trajectory which saw him go from one of the greatest antagonists Goku has ever faced to one of Goku's most crucial allies is one of the most notable examples of the frachise's well-documented heel-face-turn trope.
However, just what were the major events along Piccolo's path which caused him to undergo such a major shift in the first place? This is how Piccolo went from the literal Demon King to righteous hero over the course of two lifetimes.
Inside of Everybody, There Are Two Slugs
Before getting into how Piccolo went from the greatest evil to the greatest babysitter, it is important to come to understand a bit more about the Namekians, the race to which Piccolo belongs. While the Namekians exist in both Universe 6 and Universe 7, which are introduced in Dragon Ball Super, they actually hail from a different dimension entirely. It is once said that the Namekians were spacefarers who traded with various planets, but ceased doing so after a "Great Cataclysm" which left Grand Elder Guru as the sole survivor.
Prior to the first extinction of Namek; however, a nameless Namekian was sent to Earth by his father Katas. The fissure of the nameless Namekian results in the creation of two characters who are integral to the Dragon Ball story: Kami, who becomes the Guardian of Earth; and of course, the Demon King Piccolo.
The Cataclysm took place on Namek several hundred years prior to the events of Dragon Ball, and the planet was vastly different to the way it is now. The Namekians were highly technologically advanced at one point, and that is how they were able to be spacefarers, but it is also how the Nameless Namekian was transported off-world prior to its destruction. Prior to the Cataclysm, the vast majority of Namekians were of the Dragon Clan, the Namekian group that specializes in mystic powers – the progenitors of the franchise's titular artifacts, the Dragon Balls.
The last surviving members of the Dragon Clan are Grand Elder Guru and the Nameless Namekian; however, after undergoing the separation of his good and evil sides, the Nameless Namekian begins to forget important elements about his origin, his people, and himself. This amnesia is also partly due to the fact that his father launched him into outer space as a child, knowing that Namekian spaceships only require a rudimentary level of mastery over the spoken Namekian language.
Good and Evil
When the Nameless Namekian split into his good and evil sides, the good side became known as Kami. The Nameless Namekian arrived on the Lookout and became one of the apprentices of the aging Guardian, the other in line for the position being Garlic.
The position of Guardian of Earth was available only to those who are pure of heart, and so the Nameless Namekian expunged the evil within his own soul, splitting into Kami and the King Piccolo. Garlic was denied of the position because the Guardian knew that he only wanted the position for his selfish ends, and because there was profound evil bubbling up inside of Garlic (sound familiar?).
While Kami ascends to the position of Guardian of Earth, his evil twin King Piccolo rampages on Earth while sharing a lifeline with Kami. King Piccolo causes mayhem until his defeat at the hands of Master Mutaito and his pupils. King Piccolo is defeated by Mutaito's self-sacrificial use of the Mafuuba Technique (Evil Containment Wave); however, since the technique only seals away the target rather than kill them, King Piccolo is unleashed upon the world once again when he is released by Pilaf at the end of the 22nd World Martial Arts Tournament.
Ancient Ogre
Free for the first time in over 300 years, King Piccolo quickly descends upon all the martial artists in the world to ensure that there is no one alive who could make use of the Evil Containment Wave, the first of many victims being Krillin. King Piccolo embarks on a quest to gather the Dragon Balls to wish for his youth back, killing Master Roshi, Chiaotzu and, once his wish is granted, King Piccolo murders the eternal dragon too, preventing anyone from using the Dragon Balls after him. In a tough battle against Goku, King Piccolo finds himself thoroughly defeated as all of his deformed sons are dead, and he is mortally wounded.
Before dying, King Piccolo spits up a giant egg containing his last son and reincarnation so that someday in the future, he will have his revenge. In doing so, King Piccolo maintains the lifeline between Kami and "himself", meaning that his own death does not result in the death of the former – which then, consequently, the Dragon Balls remain operational. This enables the revival of the scores of people who died during the reign of the evil Namekian.
Ma Junior
Despite being the reincarnation of the evil King Piccolo, Piccolo Jr., known simply as "Piccolo" is not the exact same person as his father/self. Introduced as proud, sadistic and even cruel in combat, Piccolo proved to have subtle differences from King Piccolo during the events of the 23rd World Martial Arts Tournament.
While he did harbour ambitions of world domination and destroying Goku, those were largely the remaining desires of his father and seemed to fade with time; however, being defeated by Goku in the tournament added to his enmity. Piccolo shares King Piccolo's intelligence and all of his abilities; however, given that he is the son of one half of the Nameless Namekian who was sent to Earth as a child, Piccolo's understanding of his own heritage, history and identity are largely reconstructed from his extensive association with Goku and the Dragon Warriors, as well as the passage of time.
As Piccolo fights alongside Goku and his friends, he comes to respect them and even Goku, despite considering him an enemy for the longest time. The beginning of Piccolo's development into one of Earth's noblest defenders begins when he kidnaps Gohan, as his mentoring of the young boy as part of his ambition to destroy Goku takes a different path.
This path culminates in Piccolo's interception of a ki blast from Nappa meant for Gohan. He states that his relationship with Gohan taught him about friendship and love, and to this day, it remains the most profound catalyst of change in Piccolo's character.
Shin and Nail
Piccolo has no connection to his home planet prior to the Frieza Saga, initially believing himself to be of demonic origin. He soon comes to learn of his Namekian origin, and joins the fight against Frieza to avenge the massacred Namekians; however, another profound event in Piccolo's path to self-discovery occurs on Namek: he meets Nail. Piccolo's fusion with Nail meant that he could finally operate in full understanding of his origins, not to mention that he was granted a huge power boost. The Namekian ability to fuse was shown again during the Android Saga when Piccolo sees no other choice but to become whole again in order to defeat Cell. Kami could sense very little evil within Piccolo at this point, and after their rejoining into a single being once more, the evil within Piccolo completely dissipated.
After his fusion with Kami, Piccolo's demeanour is still largely the same; however, he becomes more expressive of his care for living things and his friends. He tries his hardest in various battles since then to prevent Earth's destruction and the emerging of great evil, with his most recent exploits seeing him infiltrate the newly-revived Red Ribbon Army to gather intel, team up with Gohan again, and even raise the new generation through training Pan. It's clear that despite being born to fulfill the evil agenda of King Piccolo, Piccolo Jr. has always been his own person, one capable of doing good in this world. He just needed to find himself, that's all.