Actually, sometimes slavery was ended by convincing slavemasters that it was wrong. Many slave owners experienced ethical epiphanies in the decades leading up to the Civil War, and freed their slaves. If you care about individuals, you must appreciate helping as many people as possible by persuasion, no matter how few. And to continue the analogy, it was a few rich unyielding owners that responded against this positive trend with violence. That is, the bad guys started the war, not the good guys.