- You may be looking for one particular killjoy.
Killjoy was a word generally applied to people who ruined other people's fun or happiness.
On Terra Alpha, the word was a cultural stigma, applied to people who rebelled against Helen A and her Happiness Patrol, being sad and wandering the streets alone rather than being happy and with others. One such killjoy was lured by Silas P and was executed by the Patrol. (TV: The Happiness Patrol)