Explanation: During the US Civil War, many troops from the North were ambivalent about slavery, or against it in only general terms.

As Union forces entered the slaver states of the South, and encountered more former slaves, their stories, and their scars, the abstract opposition turned, in many men, from abstract and detached, to a burning hatred of the entire system of slavery imposed on their fellow man. Some choice quotes from Northern soldiers taken from the original OP’s explanation:

“With these old people were a few small Negro children from two years old up to six or seven years. There was no money in these poor old worn out slaves and the cruel and barbarous master had abandoned them to their fate. As I looked at their worn out hands and fingers and bodies I thought of the long cruel years of bondage while under burning suns and in cold and heat they had labored for this hellish system of human slavery and now in the close of nearly a century they were only a few hours from absolute want and the misery of hunger.”

“Oh! hardened depraved man, to think of owning property in men, women and children. Man, the last and noblest work of God, possessed of body, mind and soul, of passions, love and hate. All bought and sold by man for a concideration and computed in dollars and cents. Is there a just God, and will he always see his creatures thus oppressed, and not send retributive justice with a sword of vengence to teach traitors their duty, and punish them for passed offences?”

“I thought I hated slavery as much as possible before I came here, but here, where I can see some of its workings, I am more than ever convinced of the cruelty and inhumanity of the system. It has not one redeeming feature.”