Artificial Human #17, Android 17 in the English adaptations, is a fictional character who first appears in the Dragon Ball manga created by Akira Toriyama, as well as Dragon Ball Z and Dragon Ball GT. Originally a human, he is constructed into a cyborg by Dr. Gero like his twin sister. 17 is a mass murderer in Future Trunks' timeline. Number 17 is not an android created by Dr. Gero. He and his twin sister #18, were once human. How could they be twins if they weren't once human? Toriyama said Gero found them as a couple of hooligans. They existed before Dr. Gero, therefore, they are not robots. An android is a robot and a cyborg is a human with robot parts in them. Considering what he can do, #17 is probably about half human. Now as to the motivation behind #17 killing Dr. Gero, that's subjective. While they didn't necessarily like being turned into cyborgs, once it was done, it couldn't be undone. It was Dr. Gero's strangle hold on their freedom, as represented by the "controller" device, that inspired #17 to kill him. Threat + Rebel = dead Gero. They were both rebels. You should never try to put a leash on a rebel. Later, #17 and #18 drive around in a van with Android 16, looking for Goku. When they finally get to the Kame House, Piccolo tells them Goku is not there. He refuses to tell them where he went to, and thus Piccolo challes #17 to a battle. As they fight, Piccolo's ki leads Cell (an evil insect freak spawned by Dr. Gero's computer who needs to absorb #17 and #18 to reach its perfect form) to the battlefield. He offers #17 to surrender so he can become a part of greatness. #17 refuses, and is defeated during the battle.
Number 17 is killed when Cell explodes, but is wished back to life along with the other people on Earth who had been killed by Cell. #17 has his explosive device removed by Kuririn with the second wish. The last time we see #17 in DBZ is when he is in the woods donating energy for Goku's Genki-Dama. Considering the fact that he is alive during the Kid Buu saga serves as a testament to his good soul: the wish that revived him was one that Vegeta made asking for only the good people on Earth to be revived when it was restored (Kid Buu had previously obliterated the planet in a single attack).*Wikipedia