John Locke would be against the war but if he had to he would side with the north because he is believes that everybody deserves rights so by giving African slaves rights, the country can be equal. He writes,
First, They are to govern by promulgated established laws, not to be varied in particular cases, but to have one rule for rich and poor, for the favourite at court, and the country man at plough. Secondly, These laws also ought to be designed for no other end ultimately, but the good of the people. Thirdly, They must nit raise taxes on the property of the people, without the consent of the people, given by themselves, or their deputies. And this properly concerns only such governments, where the legislative is always in being, or at least where the people have not reserved any part of the legislative to deputies, to be from time to time chosen by themselves. Fourthly, The legislative neither must
These "rules" he defines show his position on freedom and probably what he would have said while siding with the North. He really wouldn't like the war but would stick with his beliefs.