A common refrain I hear from confederate apologists is that the north only cared about slavery because it opposed their primary economic methods (wage labor), and that non slave owning farmers were afraid of southern plantation owners with massive amounts of wealth and slave labor would take over all the good land in the frontier.
I can see that being true, but also, guys like John Brown were hanged for their beliefs, so surely their was a good amount of the opposition that was due to believing it was wrong.
I'd certainly disagree with that view. If you look at the Republican platform, any of Lincoln's speeches, other political speeches by anti-slavery politicians (e.g. Charles Sumner), newspapers, and literary works (e.g. Uncle Tom's Cabin) the argument is consistently that slavery was wrong. There is an immense amount of such material. I am unaware of any material that argues slavery should be ended for reasons that would financially benefit the north.
In the Northern States, Lincoln was opposed by the Democratic party and their platform was always trying to find peaceful compromises with the Southern slave states and to agree to their demands. A lot of northerners did support the Democratic party and they were not advocates of freeing the slaves. They didn't want slaves to be freed and to move to the North and compete against themselves as cheap labour.
During the war, mostly Irish immigrants in NYC revolted against the draft and they lynched some blacks in anger against the war. Not only didn't they want to fight a war that they had no interest in but many of them were of that same view, that free blacks would just take all the low end jobs that they wanted.
Northern banks, fabric mills, manufacturing, and shipping companies greatly profited from the southern slave economy. Northern business leaders were doing extremely well and it was not in their financial interest to get rid of slavery.
So, in summary, it was not in the financial interest of Northerns to free the blacks.
Abolitionist speeches from men like Fredrick Douglas and literature like Uncle Tom's Cabin helped drive people to believe slavery was wrong. Bleeding Kansas and The Fugative Slave Act further drove many neutral northern individuals to a position that was opposed to slavery for moral reasons alone.
As time progressed, most Northern Democrats switched to the Republican party. That resulted in the election in Lincoln and that resulted in the South seceding from the Union. They knew that in the long term, staying in the Union would probably spell the demise of slavery.
In regards to "all the good land in the frontier", the argument wasn't about the land. There was an immense amount of land. The argument was about the count of slave vs free states and thus the balance of the Senate. The slave states needed to grow to the west to preserve their balance of power in Congress. So, they broke agreements and forced Northerns to accept several compromises to extend slavery into those territories. They did this by forcing votes to occur in those Western States that were previously going to be free. They argued for "States Rights", the right for Western Territories to vote yes or no on the question of slavery.