Why did people fight against slavery?

Answer 1

From the outset, many colonists believed that the ideals of the Declaration and the Constitution were incompatible with keeping a people in bondage for generations, without equal rights and freedoms. But to keep the US together, the South was allowed to maintain slavery. Within a few decades, the movement for abolition became stronger, and the Southern states (dependent on this relatively cheap labor) chose to secede from the Union rather than have their economies ruined.

In the Civil war, the North was not principally fighting against slavery, only secession. That was why Lincoln had to issue the Emancipation Proclamation - to turn the war into an official crusade against slavery, in order to keep the British and French from sending aid to the enemy. (It would have made them look pro-slavery themselves.)

Answer 2

Simple. It was wrong.