The very dopple-ganger nature of a Face Dancer implied that a Dancer was created to "take the place" of a human that the Dancer would be imitating. To make the ruse complete amongst family, friends, and compatriots; the original human would therefore have to be killed, and the body disposed of. To humans, this was an inherently wicked act, and spawned the above defined phrase.
Erasmus and the Enhanced Face DancersEdit
Even the Thinking Machine ruler, Erasmus, as he came across the Enhanced Face Dancers send out by the crazed Tleilaxu Master Hidar Fen Ajidica, believed this to be true. As he improved upon the Dancers mimicking abilities, Erasmus installed a "kill switch" in the genetic make-up at the base of their skull of all of the Enhanced Face Dancers, before sending them back into human-kind to replace key leaders. This kill switch enabled the Enhanced Dancers to commit suicide instantly when "found out" and cornered by suspecting true humans.
On Synchrony, on the day of Kralizec at the end of the age, after human-kind and the Thinking Machines had severely exhausted each other's resources in their years-long battles, Khrone, leader of the Enhanced Dancers, sprang their decades-long trap. Khrone declared that not only did the Enhanced Dancers not serve Machines, but that they were going to rise up, and rule and dominate humans and machines alike.
Activating the "Kill Switch"Edit
After Khrone had announced his intentions, Erasmus strode around the Cathedral capital of Synchrony and talked about the inherent evil nature of all Dancers to the human leaders assembled. He talked about the unfaithfulness of Dancers and talked of humans had coined the above defined phrase. As Khrone and his entourage, and millions of Enhanced Dancers across the galaxies moved into to fight humans and machines, Erasmus activated the Machine's Tachyon net. Through the tachyon net, Erasmus activated each "kill switch" in all Dancers throughout the galaxies, killing every Enhanced Face Dancer immediately.