John Brown was an extremist. This isn’t to demonize him, he was obviously in the right to fight against slavery, but unfortunately systemic progress happens within the system. Extremists can help create the environment for that systemic change to happen, but they can also stymie it depending on their methods and the prevailing ideas of the time. John Brown’s contribution was important, but he was never going to be a reasonable candidate for national office.
Calling Lincoln a centrist because there existed a more radical wing of his party is nonsensical. That’s like saying 1 is not a positive number because 2 is further from 0. Not a single southern state supported him, to the point that his election triggered a war over slavery. He was very firmly an abolitionist, which was certainly a more popular position than it had been previously in American politics, but was far from centrist.