I believe it was an act of both terrorism and moral courage. The armed insurrection proposed by John Brown (1800-1859) was considered a horrible and treasonous act in the South, where a slave revolt would have shaken the foundations of Southern economics. In the North, Brown's action was thought to be just and heroic, a moral decision that resulted in the sacrifice of his life.
From any point of view, it is hard to justify his prior acts of outright murder in Bleeding Kansas, where in several instances, he took men from their homes at night and killed them with swords and sabers. Brown no doubt considered his acts not only a religious mission but a military one as well. His planned uprising via his Harper's Ferry raid reflects a slightly more noble intent, though most of his actions were of a totally illegal nature in any state. Although Brown's desire to free the slaves was lauded by millions around the world--both then and still today--it is hard to justify murder and terrorism no matter the nobility of the cause. Brown's actions have been compared to those of Osama Bin Laden and Timothy McVeigh, yet he has been revered by the likes of Frederick Douglass, Walt Whitman, John Greenleaf Whittier, William Lloyd Garrison and Victor Hugo. Malcolm X went as far as to say that Brown was the one white man that he would have allowed to join his Organization of Afro-American Unity. Abraham Lincoln called Brown a "misguided fanatic"--perhaps the most accurate description of John Brown.
(BTW, I happen to be a descendant of John Brown--he married one of my grandmother's cousins--so I try and look at my ancestor with an open mind.)