The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. One who kills a tyrant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tyrannicide
    n 1: killing a tyrant