President George Washington personally commanded 12,000 state militia troops that occupied parts of Pennsylvania to put down the Whisky Rebellion. No combat occurred, but they were prepared for it if it arose.
It is possible that William the Bastard was part of the crew that hacked Harold II body apart.
President Madison personally but briefly commanded American troops at the Battle of Bladensburg during the War of 1812, and barely escaped from British troops; the White House was burned soon afterwards. President Lincoln personally ordered U.S. warships to shell targets in the Hampton Roads area during the Civil War. Neither pulled a trigger himself while in office.
The Witch-king of Angmar and King Eärnur of Gondor? Though no one actually witnessed that duel so it’s not certain if it actually happened.
King Theoden slaying the Haradrim king in the Battle of Pelennor Fields?
Duke Paul Atreides doing in the Baron Feyd-Rautha Harkonnen in a knife duel? Despite not being kings, by our standards, both were heads of state.
Byzantine emperor Phokas was personally decapitated by Heraclius, who then took over the throne.