"The Man from Earth", a short story by Gordon Dickson, first published in Galaxy Magazine, June 1964, available at the Internet Archive. The story matches your description, except that it's not the ruler who asks "what was he?" at the end.
Step by echoing step he walked the long length of the hall and passed through the towering archway into the bright day outside. He made it as far as halfway down the steps before, inside the hall, the Director's finger lifted, the message of that finger was flashed to the ranked guards outside, and the black rods shot him down with flame in the sunlight.
On the balcony above, overlooking those steps, Avoa stirred at last, turning his eyes from what was left of Will and looking down at Khal Dohn beside him.
"What was . . ." Avoa's voice fumbled and failed. He added, almost humbly. "I am sorry. I do not even know the proper pronoun."
"He," said Khal Dohn, still looking down at the steps.
"He. What did he call himself?" Avoa said. "You told me, but I do not remember. I should have listened, but I did not. What did you say—what was he?"
Khal Dohn lifted his heavy head and looked up at last.
"He was a man," said Khal Dohn.