Dragon Ball Z: Buyuu Retsuden/Android 18

Android 18 is the future wife of Krillin and the brother of Android 17. Formerly a runaway along with her brother, she was turned into an android by Dr. Gero. Not wanting to follow orders, they killed their creator and then terrorized the planet (going as far as to heavily injure Vegeta) before 18 became an ally of Goku after the defeat of Cell. In the alternate future, 18 and 17 destroy civilizations and kill many including Gohan. This prompted Trunks to go back in time and try to stop the two from carrying out the genocide.