One of the more memorable antagonists introduced in Dragon Ball Z was Android 17, the twin cyborg brother of Android 18, created by Doctor Gero as part of the mad scientist's plot to kill Goku. As a melding of organic and cybernetic parts, Android 17 and his sister were introduced as the ones responsible for Future Trunks' post-apocalyptic timeline, as they had wiped out most of humanity after Goku's unexpected death by heart disease. While Android 18 would repent from her wicked ways and find love with Krillin, 17's fate was more ambiguous as he went into seclusion, last seen - briefly - at the end of DBZ helping Goku raise energy for a Super Spirit Bomb to destroy Kid Buu.
Fans have observed two very different futures for Android 17 since the conclusion of DBZ. One is official, ongoing canon while the other has been retconned from continuity altogether. The canonical continuation of 17's story saw the character become a full-fledged hero, ultimately responsible for saving the entire Dragon Ball Multiverse in Dragon Ball Super. Picking up from his final appearance in DBZ where he was seen confronting poachers in the wilderness, 17 was revealed to have become a park ranger, protecting rare species on a remote island. Since the Cell Games, he had even formed a family of his own. Recruited by Goku to participate in the Tournament of Power as a Universe 7 representative, fans also learned that 17 has grown as strong as Super Saiyan Blue, playing a pivotal role in defeating the other universes' champions. In fact, it was him that used the Super Dragon Balls to restore the erased universes.
Unlike Super, the original DBZ sequel, Dragon Ball GT, presented a much less noble and considerably more tragic fate for Android 17. As it turned out, Dr. Gero teamed up with the evil Doctor Myuu in Hell after the latter was betrayed and killed by his own creation Baby, GT's first serious antagonist. The two mad scientists built an exact replica of Android 17 - one with much more sinister, violent programming than the original. The alternate Android 17's creation began affecting the personality of his predecessor, who mysteriously resurfaced to ambush and defeat Trunks without any explanation. With both versions of 17 interlinked, Gero and Myuu used the two's connection to open a portal between Hell and Earth, with many of the deceased Dragon Ball villains escaping while Goku was trapped in Hell with Frieza and Cell.
After killing Krillin following 18 rejecting an offer to rejoin him, 17 betrayed his creators. The Androids 17 merged to form Super Android 17, a significantly more powerful composite warrior. Visibly larger and more muscular than his original form, 17 had a receptor on his chest that allowed him to absorb even the most devastating energy attacks to add to his power, similar to Gero and Android 19. Easily defeating the assembled Z Fighters, Goku returned to Earth to face Super 17 only to learn that not even his 10x Kamehameha Wave at Super Saiyan 4 couldn't overload him. While Android 18 distracted her brother, Goku took the opportunity to physically charge through Super 17 to destroy him once and for all.
Despite the series no longer being canon, Android 17's return to villainy in Dragon Ball GT far more potent than the character's heroics in Dragon Ball Super. Both series keep 17's whereabouts and activities a mystery but GT is truer to the character, showing 17 as a loner, unable to forge connections beyond his sister, as she left to start her own family. There's a melancholy that underscores his dark turn, especially with him lashing out and killing Krillin as his first target. Even more tragic is the heavy implication that 17's violent actions are against his will the entire time, influenced by his doppelgänger literally straight from Hell. Upon Super 17's destruction, Android 18 speculates that, in his final moments, her brother purposefully made himself vulnerable to Goku's attack, hoping to destroy his villainous counterpart along with himself.
In a lot of ways, Dragon Ball is full of antagonists undergoing heroic redemptions, from Yamcha and Tien in the original series to Piccolo and Vegeta in DBZ. 17's arc in Super is certainly keeping in tradition with this, what with the revelation that the character hasn't pursued his wicked ways for sometime...but it's just a bit too convenient. 17's arc in Dragon Ball GT reflects both his villainous origins and tendency toward violent betrayal while explaining why the character had grown so much stronger in the interim. And, at the end of the day, 17 still gets his redemption - albeit, at the cost of his own life - to stop a greater evil menacing the Z Fighters. Android 17's ultimate sacrifice to stop the monster wearing his face is certainly one of GT's most under-appreciated storylines.