Future Android 17's History
Background
Future Lapis and Future Lazuli's lives were exactly the same as their main counterparts: they were notorious delinquents, until at some point in time, Future Dr. Gero kidnapped the siblings and then transformed them into Androids 17 and 18, programming them to hate humans as part of his world domination bid; they rebel against him and the doctor is forced to shut them down, and during this time he is focusing on his ultimate android creation, so one day it would absorb them and become a perfect fighting machine. Six months after the death of Goku in Age 766, Future Android 17 and Future Android 18 are released by Future Dr. Gero to kill Goku as part of his plot for revenge. According to Cell's flashback, Future Android 17 and Future Android 18 are wandering through Dr. Gero's Laboratory as Gero is working on his "ultimate android". Future Android 17 counters Future Dr. Gero by saying he is the ultimate android while Future Android 18 knocks over some glass. Gero attempts to use his remote, but Android 17 decapitates him. As Gero's decapitated head says that his "ultimate android creation" will absorb them, Future Android 18 blasts Gero's head.
Dragon Ball Z: Cell Saga
In "Ghosts from Tomorrow", Future Android 17 makes a cameo when Future Trunks mentions to Cell how the Androids have caused devastation and destruction in his timeline killing any vulnerable human that gets in their way during a flashback. The Androids then proceed to kill the Z fighters one by one, leaving only Future Gohan and Future Trunks as the only warriors left to face them.
Future Gohan tells Future Trunks to stay back as he's about to fight the Androids. In the flashback, Gohan battles with Android 18 first, and is punched several times before he is kicked into a nearby building. Future Trunks then steps in for Future Gohan as Future Android 18 prepares to fire a blast at Future Gohan but is quickly blasted away after he slashes a few strands of her hair with his sword. Future Gohan then knocks away Future Android 18 but Future Android 17 quickly intervenes knocking Future Gohan into a rockpile. Future Android 18 then fires a blast at Future Gohan but Future Gohan quickly dodges it, flies up and uses his Masenko, quickly blinding the Androids.
After the attack, the Androids quickly emerge out of rubble. They realize that Future Gohan and Future Trunks have hidden somewhere within their vicinity, so they decide to blow up the area to draw them out of hiding. The next scene shows Gohan and Trunks charging at the Androids, but nothing beyond this is seen other than a large explosion and Trunks' narrative describing his mentor's death and how he was the only one to survive against them forcing his mother to invent a time machine so he can travel back in time to alter the events that happened in his future.
Trunks finally returns to his own time after the defeat of Cell in the main timeline, and with his training in the Hyperbolic Time Chamber, knows he has more than enough power to finally kill the androids. While the androids are attacking Parsley City, Android 17 ends up murdering a boy that Android 18 thought was cute, causing her to become very upset at 17 and not talk to him, and later attempts to kill an old man, the boy's father, after the latter tries to shoot 17 in order to avenge his son. In the Japanese and Kai dubs, as well as the manga, the source of 18's anger is because she lost at a video game.
Before he can kill the man (in the anime only, in the manga, he kills the old man just before Trunks arrived, either by shooting the man with his own pistol [original manga], or simply punching him out [VIZ edition]) Trunks arrives declaring that he is going to cause their end. Android 17 insists on just playing with Trunks instead of killing him, but Android 18 furiously attacks him, only to prove no match for his new found power. Android 17 soon joins the fight, and the two attempt to attack the mighty Super Saiyan. Trunks effortlessly dodges and blocks both of their hits, and eventually singles out Android 18, who he obliterates with a powerful attack. Android 17 remains paralyzed by fear and in disbelief. He is at first completely disbelieving that his sister just died, then his horrified sorrow turns to anger as he vows to make Trunks pay for the deed (in Kai and the manga, he asks Trunks how he became so strong without importing that his sister had just been killed). Trunks, however, asks 17 if the other people they had killed mattered, if their lives had been worth something. Trunks then charges Android 17 and knocks him down with a powerful kick to the jaw. Showing no mercy, Trunks blows the prone 17 to bits with a powerful energy blast, killing the evil android, and ultimately ending the threat of the android twins, and finally avenging Gohan, although there is still one more android to stop: Cell.
