Sadness
From Tardis Wiki, the free Doctor Who reference
Sadness was an emotion.
The Vardy were programmed to keep the colonists of Gliese 581d happy at all times. When one of the colonists died of natural causes, her friends and family began to grieve. The Vardy, reading grief as the opposite of happiness, treated it like a virus and began killing the colonists as to them, eliminating sadness meant eliminating sad people. (TV: Smile)
