Quote from The Watchtower, March 15, 2015, Page 23:
In the illustration of the faithful and discreet slave, Jesus spoke of an evil slave who beat his fellow slaves. Jesus was not there foretelling that an evil slave class would arise. Rather, he was warning the faithful slave not to display the traits of an evil slave. Similarly, in the illustration of the ten virgins, Jesus was not indicating that half of his anointed followers would be like the five foolish virgins. Instead, he was warning his spiritual brothers about what would happen if they lost their sense of vigilance and did not prove to be prepared.