What's scary about terrorists like John Brown is the cold soundly logical tone of their words -- that is, if you accept their basic premises. Brown believed that slavery constituted a sin against God over all others, and that blood shed in an effort to bring slavery to an end somehow represented a down payment towards eventual settlement of the account.
Tens of thousands of people across the North bought into Brown's reasoning, at least to the point that his actions had the best intentions of any true hero. And the "Battle Hymn of the Republic" represents Julia Ward Howe's paean to the holy martyr whose actions she described as "heroic", John Brown --
I have read a fiery gospel writ in burnished rows of steel:
"As ye deal with my contemners, so with you my grace shall deal;
Let the Hero, born of woman, crush the serpent with his heel,
Since God is marching on."