In reading New Hampshire letters, I found many where soldiers now came at the issue as a practical matter rather than a moral issue. It wasn't so much that freeing the slaves was right as it was that freeing them would hurt the rebel cause and end the war faster.
Take, for example, the opinion of a 14th New Hampshire soldier who called himself Lauren E. Bent (his real name appears to have been Lauren E. Gardner). Bent was a 20-year-old private from Winchester, N.H. I found his letters at the Historical Society of Cheshire County and quoted him in Our War.
Here is what Bent wrote to his mother and sister back home on Feb. 15, 1863:
On Sept. 19, 1864, at Winchester, Va., during the 14th New Hampshire's biggest battle of the war, Private Bent was severely wounded. He died the next day.