“The abolitionists were actually a small minority of the Northern population. Most of the North didn’t care one way or the other.”
Very true but with one notable exception —Massachusetts harbored a lot of abolishionist sentiments. Harriet Beecher Stowe, William Lloyd Garrison, Frederick Douglass and much of the manpower that became the 5th Massachusetts Infantry Regiment.
Puritan influence, in my opinion. They had the same "I'm doing the Lord's work!" fanaticism when they were rooting out all of those witches up in Salem too.