Question

Why did the Fugitive Slave Act anger Northerners?

Answer 1

It also forced people in northern states to take sides on the issue of slavery.

Explanation:

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.