The History Books try to claim that William Bruce Mumford was the only person to be executed for treason, following an act of taking down an American Flag the Union Army raised in New Orleans in 1862.
But Louisiana had already seceded, and was not a part of the Union.
So that claim has no merit.
On the other hand, John Brown had committed murder and was provoking a slave revolt in 1859, before any states seceded.