Although John Brown is hanged quite early on in Benét's poem, he nonetheless remains its guiding spirit, an almost constant brooding presence. That he does so is revealing of the poet's attitude to this hugely controversial figure in American history.
Damned by some as a dangerous, deluded fanatic and hailed by others as a hero, Brown's reputation remains a source of contention. However, for Benét, there's no ambiguity whatsoever: Brown was a great man who changed the course of American history for the better.
Brown may have been a failure in his mediate aim of freeing the slaves, but in the overall scheme of things, from the standpoint of eternity, he achieved great things according to Benét. Due to the power of his example and through the force of his remarkable personality, Brown was able to create what Benét calls "a crack in Time," a disruption of history that in due course gives rise to irreversible change.
On Benet's interpretation, Brown is one of those unique historical individuals who radically changed the course of history, altered "the actual scheme of things" to the extent that the United States would never be the same again after Brown's quixotic raid on Harper's Ferry.
Benet admits that John Brown had "no gift for life," but for him, that is unimportant. What is important is that Brown "knew how to die." This can be seen in his last speech to the court in which he was tried and sentenced to death. In these inspiring words, Brown showed a willingness to sacrifice his life in order to further the ends of justice, to put an end to slavery once and for all.
Brown may have died not long after, but his words and his example lived on after him, inspiring others to take up the noble cause to which he had devoted his life and for which he was prepared to die. That, more than anything, is why Benét admires him so much, even though he concedes that John Brown had his faults.