How did the fugitive slave law backfire?

It was launched in a bad spirit as part of the ill-starred Compromise of 1850.

To get the South to agree to allow California to enter the Union as free soil, Congress had to make a big concession in favour of slave property. The Fugitive Slave Act was deliberately designed as a dramatic gesture to impress the slave-owners. The general public must now report anyone who looked as though they might be a runaway, on pain of a heavy fine.

The Northern public strongly resented being treated as unpaid slave-catchers, and the Abolitionists reacted in fury, setting-up the Underground Railroad, a system of safe-houses by which slaves could be smuggled into Canada. Harriet Beecher Stowe wrote 'Uncle Tom's Cabin' as her personal response to the Act, and it sold in its hundreds of thousands, recruiting many more people to the Abolitionist cause. This publication alone probably did as much as anything to start the Civil War, as Lincoln wryly commented.