Given Lincoln’s stated views that blacks should not be considered socially equal to whites,
and given that the Confederacy’s founding openly stated that those in power considered blacks inferior,
and given that the North acted essentially as a occupying force in the South during Reconstruction with the aim of keeping the South from rebelling again (taking over industries and creating harsh conditions for most of the whites, including poor education for both races),
one can easily imagine that emancipating the black slaves who formed a quarter to a third of the population was a covertly punitive act against the white population—clothed in noble language about human dignity.
Imagine whatever you like. But in the 19th century opposition to slavery didn't require commitment to an idea of racial equality. Lincoln said:
I agree with Judge Douglas he is not my equal in many respects-certainly not in color, perhaps not in moral or intellectual endowment. But in the right to eat the bread, without the leave of anybody else, which his own hand earns, he is my equal and the equal of Judge Douglas, and the equal of every living man.
That's not a sentiment we can approve of today, but at the time it was an advance in our ideas of human dignity.
Moreover, the thing about war is that people are busy trying to kill people and win the war. Other goals, good and bad, come second. So it's always possible to dismiss people's other motives and actions as unimportant. Until you actually get into a war and finally understand the mindset.
It was also a grab at raw political power. Right after the civil war, all the whites were denied the right to vote because they had been in "rebellion." Only blacks could vote, and of course they were going to vote strictly Republican.
This gave Republicans in Congress a huge majority in the congress, and with it they could do whatever they wanted with the power of the US Government.
Always follow the money and the power. It will tell you the truth when everyone else is trying to mislead you about stuff like "human dignity."