1.  regicide(n = noun.person) - someone who commits regicide; the killer of a king;
is a kind of killer, slayer

2.  regicide(n = noun.act) - the act of killing a king;
is a kind of execution, murder, slaying

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]

regicide, n.
1 a person who kills or takes part in killing a king.
2 the act of killing a king.

Derivative:
regicidal adj.

Etymology:
L rex regis king + -CIDE

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