He is one of the Cyborgs created by Dr. Gero for the purpose
of killing Son Goku. In Future Trunk's world, he's simply an evil individual
with no regard for human life. Along with #18, he killed most of the world
including Piccolo, Vejeta, Yamucha, Tenshinhan and even Son Gohan, leaving
Future Trunks as mankind's only hope.
He is quite a different character in the "present" world. Although his
character is not evil, his big ego got the best of him. Believing that no one in
the universe is stronger than him, he could not accept that fact that he was
unable to defeat Cell and fell an easy victim to Cell, thus contributing to
Cell's Development to 2nd Stage