Or invading IRAQ for their WMDs
I don't know an awful lot about the civil wars, but I know about the context, that it was a period of consolidation for US. In that light, the slavery could just have been a casu belli for weightier reasons.
There were many many points of view during the CW what about what there was to fight for/against. I'm not familiar with your ideology but we only need another adult for a serious discussion and I'll assume you to be one and give it a shot
First things first. I'll try approaching this from the perspective of those who lived during those times. That means I'm not going to pontificate against slavery because the absolute worst criteria any situation can be judged by is that which didn't not exist at the time.
Second is an understanding of what slavery was, and that's an economic system. No one brought Africans here to oppress them. The economy of the south was mostly agrarian. Cotton was King. That and other farm crops sustained the South economically and provided the North with it's needs. The North was more of an industrial society and slavery made no sense. It was unnecessary.
IMO this is the key to understanding the people who took sides during the Civil War. Note that what follows is a generalization and "all generalizations are bad" holds. Nevertheless I believe this applies to the collective for our purposes.
The South believed that slavery was necessary for their economic survival. Further, they believed that slavery should be an option in new territories and should not be limited by those who have no understanding of what would be agricultural areas.
The North had a chance to distance themselves from slavery once it became unnecessary. They saw the abuses and weren't happy about them. Let's face it, slavery is harsh and brutal in most cases, and the South was not a place that could boast of it's humanity.
So the North felt a moral imperative to stop or at least contain slavery, and the South felt that it's very survival required it.
Some might stop there, but it's not that simple. Complicating this was the feeling by some that the Union was the highest form of government possible and therefore had to be kept united at all costs. Others felt that the Union was only binding as long as it's participants were willing to abide by it's conditions. If they ever came to be seen as onerous, then leaving was an option. After all the US left under hostile terms for that very reason.
Competing economics, needs, and political outlooks which were not unrelated but came to create a complex and varied perception of moral and philosophical considerations.
Many factors created a climate of hostility fear and distrust. The War was inevitable, but the first match to the fire was economics.