What side was truely right?

Lets just put it as simply as I can. Under the United States Constitution prior to 1861, no part of the federal government, or any other State, or Group of States, had the authority or right, to tell or dictate, to any other State, or group of States what that State's internal policy or politics were to be. Each State was an independant political subdivision. Under the 9th and 10th Amendments the majority of powers to govern the people of the United States had been given to the individual States to govern as the state legislatures decided. Hence "States Rights".

States Rights is why the Southern States seceeded. The threatened overreach of a Federal Government, backed by other states, to dictate to other states what each states could do and shouldn't do. That was a direct threat to the independence of the individual states that had be the very cause the American Revolution had been fought for in 1776. The freedom to decide within your own state what was in the best interest of the people of that States.

The present day overreaching Federal Government of today that tells you what you can and cannot do in every part of our life is the direct result of of the southern states defeat in 1865 and the death of the United States Constitution as written in 1787.