most beautiful most horrible Christmas Carol of all / the Slaughter of the Innocents (disambiguation)

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This is, I think, the oldest continuously sung English Christmas Carol. It was written and performed by a Guild in the Coventry Cycle of religious plays, on the church/cathedral steps. They (Alison Moyet here) sing of the monstrous calamity Christians call The Slaughter of the Innocents. Herod the King ordered all boys younger than two in Judea slain. A prophesy foretold that if allowed to live and grow up, a boy of that age would become King of the Jews. (Herod thought Herod was King of the Jews, permanently, for life).

However, among NASA scientists, The Slaughter of the Innocents was a massive government budget cutback on funds for robot space probe exploration. The scientists thought it was morally equivalent to slaying defenseless babies. 

(I tend to agree when the government does stuff like that.)

Spoiler Alert: Send all kids out of the room before playing this Christmas Carol.