He believed not only that the slaves should be freed, but that the blood of slaveholders should be shed in atonement.
In "bleeding Kansas" he led the Ossawatomie massacre, and committed cold-blooded murders under the delusion that the sword of the Lord was in his hand.
In October, 1859, Brown, who had for some time been living under an assumed name in the neighborhood of Harper's Ferry, led a score of his followers, some of them negroes, in a surprise attack upon the Government arsenal at this place, capturing the watchmen and taking possession of the buildings.

It was his idea to get the weapons the arsenal contained and give them to the slaves that they might rise and free themselves.

Before this plan could be executed, however, Brown and his men were besieged in the armory, and here, after a day or two of bloody fighting, with a number of deaths on both sides, he was captured with his few surviving men, by Colonel (later General) Robert E.Lee, whose aide, upon this occasion, was J.E.B.Stuart, later the Confederate cavalry leader.

Stuart had been in Kansas, and it was he who recognized the leader of the raid as Brown of Ossawatomie.
It is said that Brown's violent anti-slavery feeling was engendered by his having seen, in his youth, a colored boy of about his own age cruelly misused.

He brooded over the wrongs of the blacks until, as some students of his life believe, he became insane on this subject.