Substitute one word for the italicised expression.

He was arrested for the murder of the king.

regicide

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.