I have had discussions with pro-CSA types who believe the Lawrence Massacre during the war was fully justified because it was done in retaliation for Kansas attacks on Missourians. If you accept this premise, then JB's murders on Pottawatomie Creek were equally justified.
(Personally I believe neither attack was justified.)
Here's the thing. A great moral crime was being committed against the slaves. If anybody ever had a moral right to use violence in their cause, the slaves did. Certainly they would have been far more morally justified in doing so than were American colonists in the 1770s, when they resorted to violence in resistance to what was mostly at the point possible future oppression.
If the slaves had a moral right to use violence against their oppressors, then others had a moral right to join them in their resistance, as the French assisted us in our Revolution.
John Brown was a very poor tactician, but few people at the time claimed he was crazy. Even the governor of VA, who visited him in jail, thought him a fine soft-spoken gentleman.
If you were being held in slavery in Algeria in 1800, would you have had a moral right to resist your owners with violence? If you believe you had such a right, why wouldn't American blacks held in slavery have the same right? I'm just interested in your reason why person A is entitled to defend his freedom with violence, but person B is not.
Really can one justify murdering innocent people? Does taking the higher moral ground justify cold-blooded murder?