NOUN (2)

1. someone who commits regicide; the killer of a king;

2. the act of killing a king;

The Collaborative International Dictionary of English v.0.48:

Regicide \Reg"i*cide\ (r[e^]j"[i^]*s[imac]d), n. [F.
   r['e]gicide; L. rex, regis, a king + caedere to kill. Cf.
   Homicide.]
   1. One who kills or who murders a king; specifically (Eng.
      Hist.), one of the judges who condemned Charles I. to
      death.
      [1913 Webster]

   2. The killing or the murder of a king.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "regicide":
   aborticide, fratricide, fungicide, genocide, germicide, herbicide,
   homicide, infanticide, insecticide, matricide, microbicide,
   parricide, patricide, pesticide, rodenticide, sororicide, suicide,
   uxoricide, vermicide