regicide
/ˈrɛdʒɪsaɪd/Definitions
1. noun
the act of murdering a king, monarch, or other sovereign ruler
“The court’s verdict was a shock, as the evidence pointed to a regicide, but the king’s loyal advisor was acquitted.”
2. noun
the killing of a king or other ruler, especially in a treasonous or premeditated manner
“The regicide of Julius Caesar led to a power struggle among the Roman factions.”