The one-word substitute for the italicized expression "the murder of the king" is regicide.
- Suicide is the act of intentionally killing oneself.
- Fratricide is the act of killing one's own brother or sister.
- Homicide is the act of killing another human being.
- Regicide is the act of killing a king or other sovereign.
Regicide is the most appropriate word for this context because it specifically refers to the killing of a king. The other words do not have the same connotation of royalty. | 
