The North was most objectionable to the Fugitive Slave Act. Northern abolitionists did not wish to enforce the Fugitive Slave Act; these people were against slavery, and the Fugitive Slave Act required that the North capture escaped slaves and return them to their owners. The act denied slaves trial by jury, and they couldn't testify on their own behalf; the North did not want this because they opposed slavery and advocated freedom for slaves
Hope this helps! :)