Why did the fugitive slave act anger Northerners who were anti-slavery?

The Fugitive Slave Act required Northerners to help return runaway slaves to their owners, even if they were in free states. This angered anti-slavery Northerners because they felt it violated their principles and forced them to participate in a practice they morally opposed. Additionally, it heightened tensions between pro-slavery and anti-slavery factions in the country.