tyrannicide
noun
ty·ran·ni·cide tə-ˈra-nə-ˌsīd 
tī-
1
: the act of killing a tyrant
2
: the killer of a tyrant
Word History
Etymology
in sense 1, from French, from Latin tyrannicidium, from tyrannus + -i- + -cidium -cide (killing); in sense 2, from French, from Latin tyrannicida, from tyrannus + -i- + -cida -cide (killer)
First Known Use
1650, in the meaning defined at sense 1