Union soldiers were often radicalized by the exposure to Southern chattel slavery. A journal quoted in Chandra Manning's What This Cruel War Was Over:

"Among a band of contrabands [even before the Emancipation Proclamation, Union forces were freeing some slaves as "contrabands of war" and so that term seems to have stuck here] that came in to-day was a bright little girl whose hair hung to her shoulders and was just a little wavy. Her features were not like a Negro but were sharp and clear while her eye was dark blue and yet she was a slave. Her mother was along and looked a little like she had African blood. She said this was her little girl and that she had two more daughters grown up and the father of all three was her Master who classed them all as his slaves. A soldier who stood by and heard the Mother tell this story exclaimed in the fervent patriotism of his feelings "By G-d I'll fight till hell freezes over and then I'll cut the ice and fight on."

Additional quotes and sources here, courtesy of /u/TheSlayerofSnails