Runaways weren't safe in the North because in the 1850s the North had signed the Fugitive Slave Act ---despite--- popular on the ground opposition to Northerners helping slave catchers. The on the ground opposition was so strong that juries would refuse to convict and Daniel Webster's career was killed because he brokered the compromise that signed the North up to enforce the Fugitive Slave Act. https://www.parthenonpodcast.com/history-unplugged-podcast/daniel-webster-perhaps-historys-greatest-orator-turned-virginians-and-new-yorkers-into-americans - You seem to be implicitly suggesting that Northerners did not support slave freedom. The juries refusing to convict and Daniel Webster's career, and the supermajority of Northerners who voted for the Abolitionist candidate in 1856, suggest otherwise.