Source : Webster's Revised Unabridged Dictionary (1913)

Tyrannicide \Ty*ran"ni*ci`de\, n. [L. tyrannicidium the killing
   of a tyrant, tyrannicida the killer of a tyrant; tyrannus a
   tyrant + caedere to kill: cf. F. tyrannicide.]
   1. The act of killing a tyrant. --Hume.

   2. One who kills a tyrant.

Source : WordNet®

tyrannicide
     n : killing a tyrant