Slavery was an economic necessity due to trade restrictions imposed on Southern crops, not to mention that industrialization had far more effect on Northern states, where most manufacturing was based.
Of course, that's not to say the North were benevolent gods - as we should all well know, "labor rights" didn't particularly exist until the 20th Century, and there are countless incidents documented in Northern states of corporations going as far as murder to keep their workers in line. Conversely, many slave states had laws against killing a slave without due cause.
Then, of course, there's the Fugitive Slave Act to take into consideration, passed by the United States (Union) Congress in 1850.
So basically, a slave could escape the south, get a job in a Philadelphia factory, and assuming they didn't get sent back to their master by the government, get bludgeoned to death either by the machinery they worked on, or, if they dared complain, their bosses. Better than slavery? Probably, but not the utopian promised land that a lot of people want to believe.
Here's a good article on the causes of the war: http://teachinghistory.org/his...