Slavery in the North was surely much "milder" in the North than South. The Northerners were not more humane than Southerners but their point of view to the slaves was different. There are many reasons for this.
First, the Northerners were more concerned about civil rights. The working conditions of the factory workers in the North were as bad as the working condition of the slaves. The North was more industrialized than the South. So factories were very common in North. As the number of factories increased, the people who worked in the factories increased as well. According to Call to Freedom, they worked for twelve to fourteen hours a day. But unlike in South, the Northerners had ability to organize unions. At first, unions were not successful, but they did make progress. So the Northerners became more concerned about civil rights of people and when South considered slaves as properties, they considered them as human beings.
Another reason why slavery in the North was milder was because in the North, the abolitionists were very strong. As soon as slavery was adopted, abolitionists came up. They thought slavery was morally wrong. Some abolitionists like William Lloyd Garrison demanded the immediate emancipation of the slaves in South. He published the Liberator, an anti-slavery newspaper. In one of the articles, he wrote, "In defending the great cause of human rights, I wish to derive the assistance of all religions and of all parties," and tried to make slavery and the South "the enemy of all". This press affected people in the North greatly. Even Lincoln once said that it was anti-slavery papers that brought the civil war.
Finally, the slavery in the North was milder because the North did not need as many slaves as people did in the South. As...