Answer:
John Brown was killed because he murdered a Deputy Sheriff.
Explanation:
He was tried for the murder of the Deputy Sheriff, treason, and Insurrection
(Credit for the info: https://www.history.com/this-day-in-history/john-brown-hanged#:~:text=Militant%20abolitionist%20John%20Brown%20is,insurrection%20on%20December%202%2C%201859.)