Your account by young Jennie Chambers sounds helpful, but there were Federal armories in almost every state. People dismissed Brown as being crazy but he was probably as rational as the 9-11 conspirators.
Your comparison to Charles Manson is more on target than you may realize. Allow me to ask, what exactly did Manson mean to accomplish by the murders in Hollywood?
I hope you find time to read more contemporary news acounts from 1859-60. It becomes abundantly clear that Harpers Ferry could not be explained away as a one-time event of traic nature. Southerners finally realized that certain people in the North would not shrink from boodshed to achieve their purposes, and feared more attacks like these.
Southerners also understood that new attacks might come from new fanatics who had simply been inspired by John Brown.
For what it's worth Dr. Howe was not a minister, but the Howes had abolitionist/Unitarian friends who were ministers. Dr. Howe's background included first-hand experience with insurgencies in Europe, so he was in good stead to advise Brown about leading a new insurgency in Virginia.
People will insist that secession was solely motivated by high tariffs and lofty considerations for states rights, whatever that's supposed to mean. IMHO ordinary people are far more focused on actual threats to their personal security than almost anything else.