Let's not forget that in the decades preceding the Civil War, there were various political clashes between the "North" and the "South" over slavery, especially in new states: the Missouri compromise, the Mason-Dixon line, Bleeding Kansas. These all had the theme that the North wanted nothing of expansion of slavery to new states. And yes, Lincoln didn't rally the Union behind "abolish slavery" but behind "preserve the Union", because that was easier to do. But in the end, he did abolish slavery, and in one stroke.
So while you cannot say that the North would have abolished slavery tout court, the political reality shows they did strive to abolish it gradually. The South, on the other hand, seceded with the express intention to preserve slavery in perpetuity.
I see there a clear moral difference. Even if you cannot say the North was morally unequivocally good, it was much, much better than the South, which was unequivocally evil.
(and I care less about the treason aspect: many nations were born in rebellion, including mine, and their national heroes technically tratior. The national hymn of my country even is the apology of William the Silent why he rebelled against his king).