Why did the Fugitive Slave Act anger Northerners?
It also forced people in northern states to take sides on the issue of slavery.
Prior to the passage of the Fugitive Slave Act, northerners could view slavery at a distance; it was a problem to be dealt with by southerners. Abolition was pretty far from a good deal of people's minds.
The Fugitive Slave Act made it a crime not to turn in people suspected of being a runaway slave, so it forced people to take a stand -- either by choosing to follow the law, or, as noted in the Emerson quotation that Mark B posted, to choose to disobey it. Either way, someone was taking a stand on slavery in a way that many people did not have to do in the north prior to 1850.