It's complicated -- this older answer of mine gets into some of the reasons, while this answer talks about the initial war aims of the Union, and this one by /u/Georgy_K_Zhukov talks about the role of slavery in secession and starting the war. There is no doubt that the war (on the northern side) became increasingly seen as an anti-slavery war as time went on; Union soldiers seeing the horror of the slave labor camps in the South certainly saw themselves as "armies of deliverance," to borrow the title of Elizabeth Varon's excellent book on the topic.