The Fugitive Slave Act made it so runaway slaves, even if they were in a free state, had to be returned to their owners. This also increased tension between the Northern and Southern states, as the North wanted to abolish slavery and the South wanted to keep it.
The slaves who were now in the free Northern states were very angered because of the Fugitive Slave Act, causing many to act out and protest. They thought the South was trying to spread slavery and would never stop, without the say or voting of the Nothern states.
People who were to interfere with the capturing of runaway slaves would be fined, which made many Northerners cautious, but still many wanted to help. Many would end up being recaptured and given back to their owners, or even sold back into slavery entirely. They would also be tortured, beat, and often times would be starved or killed.