Answer:
1) They weren't treated like humans, but more as a commodity, something that can be used and abused without care. In the North, people were against slavery and working to end it, so naturally, you'd go somewhere safer, where you're actually seen as a human being with feelings and unalienable rights.
2) As stated above, the North saw them as humans, offered them rights, jobs, etc. Life in the North was like other people's concept of the American dream.
3) I believe they did, they found rights and jobs, they were seen as people and not something to be sold, like a pet.