I dunno; I’m as contemptuous of the antebellum American South as anyone, but I’m still not convinced that John Brown deserves a pardon. You can be sympathetic with Brown’s ends, and even recognize that his actions played a significant role in emancipation, while being reluctant to embrace the idea that private citizens ought to be able to murder other private citizens for the conduct of activity which is perfectly legal. I appreciate that pardons are often used to recognize conduct which is laudable without being legal, but it’s difficult to forgive Brown the murder of Hayward Shepherd, who was not a slaveholder, and who likely did not support slavery. I think that there’s rather a better case for the pardon of Nat Turner, on the grounds that the institution of slavery freed its subjects of any ethical responsibility to the constituted legal authority.
In any case, I doubt that Brown would approve of even a 150 year posthumous pardon.