One side wanted to keep slaves, the other didn't. Guess which one was humanist. And which one was capitalist (hint : answer may be both).
The question isn't if the war was rightful or not, is why one was for slavery and the other wasn't. Both being capitalist, this probably isn't the answer.
My opinion goes toward humanism. Maybe wrong, I'm not a specialist in the history of slavery, but I doubt you are either.