There was a group of people called the "Abolitionists" and they supposedly worked very hard to end slavery. Their justification was that, as Christians, they couldn't stand by anymore while these atrocities were taking place. They felt that God wouldn't approve of the enslavement of humans.

Here is a powerful quote from Frederick Douglass: “Let me give you a word of the philosophy of reform. The whole history of the progress of human liberty shows that all concessions yet made to her august claims, have been born of earnest struggle. The conflict has been exciting, agitating, all-absorbing, and for the time being, putting all other tumults to silence. It must do this or it does nothing. If there is no struggle there is no progress. Those who profess to favor freedom and yet depreciate agitation, are men who want crops without plowing up the ground, they want rain without thunder and lightening. They want the ocean without the awful roar of its many waters."

"This struggle may be a moral one, or it may be a physical one, and it may be both moral and physical, but it must be a struggle. Power concedes nothing without a demand. It never did and it never will. Find out just what any people will quietly submit to and you have found out the exact measure of injustice and wrong which will be imposed upon them, and these will continue till they are resisted with either words or blows, or with both. The limits of tyrants are prescribed by the endurance of those whom they oppress. In the light of these ideas, Negroes will be hunted at the North, and held and flogged at the South so long as they submit to those devilish outrages, and make no resistance, either moral or physical. Men may not get all they pay for in this world; but they must certainly pay for all they get. If we ever get free from the oppressions and wrongs heaped upon us, we must pay for their removal. We must do this by labor, by suffering, by sacrifice, and if needs be, by our lives and the lives of others.”

Abolitionist movements are driven by altruism, but they succeeded in some places faster than others when they have less resistance by people who have economic interests in slavery.

Take Northern U.S. states for example. Most of them abolished slavery before the South did. Why? It happens that the sorts of agriculture and industries that developed in the North had little use for slaves. Slaves are expesnive to clothe, feed, and keep under control.

On Northern farms most business is easily taken care of by a small number people, and not much is gained from a lot more labor input after a certain point. Southern cotton and sugar plantations are a different story . Cotton and sugar crops are best suited to Southern climates and were in massive demand. It so happens that these crops are much more labor intensive year round but the pay off is enormous and output is much more directly related to the amount of steady labor a land owner can get put into the crop.

Likewise Northern industry there there were plenty of people willing to do the work for low-pay. Factory owners found low-paid voluntary workforce performed better in factory jobs than slaves and ended up being less expensive to maintain.

Slavery paid off in the South in ways it simply did not in Northern states and Northern Europe with their cooler, less sugar and cotton friendly climates.

The upshot is that in the North there were not many great economic interests with a stake in slavery. In the South there were. It was easy to convince ruling elites and leaders in the statehouse that slavery was wrong and should be abolished so long as those leaders were not raking in massive amounts of profits from slavery.

English and Northern European agriculture and industry were very similar to Northern U.S. agriculture and industry, and therefore ruling elites didn't have much to lose by doing the moral thing in those countries.