"During the war with England [in 1812] a circumstance occured that in the end made him a most determined Abolitionist: & led him to declare, or Swear: Eternal war with Slavery. He was staying for a short time with a very gentlemanly landlord since a United States Marshall who held a slave boy near his own age very active, inteligent and good feeling; & to whom John was under considerable obligation for numerous little acts of kindness. The master made a great pet of John: brought him to table with his first company; & friends; called their attention to every little smart thing he said or did: & to the fact of his being more than a hundred miles from home with a com-pany of cattle alone; while the negro boy (who was fully if not more than his equal) was badly clothed, poorly fed; & lodged in cold weather; & beaten before his eyes with Iron Shovels or any other thing that came first to hand. This brought John to reflect on the wretched, hopeless condition, of Fatherless & Motherless slave children: for such children have neither Fathers or Mothers to protect, & provide for them. He sometimes would raise the question is God their Father? . . ." (From a letter John Brown wrote to 13 year old boy named Harry Stearns, he's referring to himself in the 3rd person)
I think it's got a lot to do with the general lib sentiment being ultimately "whatever the general consensus is."
You'd struggle to find any lib who would disagree with John Brown's views on slavery in a broad sense - e.g. that it's very bad. Likewise, they would consider anyone living now who wanted to bring back chattel slavery as being mad - let's leave aside their tendency to write off appallingly evil viewpoints as mental illness because that's a conversation for another time.
John Brown's actions however put him against the general consensus of the south that slavery was OK - that's what makes John Brown mad in their eyes. It's not what he did or how he did it, it's that he went against the established order.