JB was an incompetent and megalomaniacal insurrectionist. The Kansas attacks were in direct retaliation for attacks on free soil men. Harpers Ferry was an attempt to start a full blown slave insurrection. It was not itself intended to have terroristic effect.
I have never heard someone give a logical explanation why the Founders were morally in the right to violently revolt against (primarily) possible future oppression, while John Brown was morally wrong to violently revolt against actual present oppression.
The oppression by Britain of Americans, as outlined in the Declaration of Independence, was in any historical sense really pretty minor. The oppression of black slaves, as of any slave, was absolute and ultimate.
John Brown wasn’t revolting against anything. He murdered seven innocent people in cold blood, using the sack of Lawrence as his excuse.