Wiktionary
sacrificial lamb
n. 1 A lamb killed as an offering to the gods 2 (context by extension English) A person who is sacrificed (not literally) for the greater good of some cause
Wikipedia
Sacrificial lamb
A sacrificial lamb is a metaphorical reference to a person or animal sacrificed (killed or discounted in some way) for the common good. The term is derived from the traditions of Abrahamic religion where a lamb is a highly valued possession.