I think that’s a very oversimplified opinion. Depending on the alternative, war can certainly be the moral decision. I don’t think you’d find many would who would describe The Unions war on slavery as Evil.
It wasn't a war on Slavery though, as Lincoln only took the extraordinary step of releasing the Emancipation Proclamation late in the war.
That’s a semantic argument. The point is that any of us could come up with an example where the decision to go to war, either offensively or defensively, is clearly not evil.
Yes, it was. South Carolina was the first state to secede; South Carolina cited slavery in their declaration of secession; the war began with secession or immediately following and because of secession. The civil war was fought over slavery.
The Union didn't fight a war on slavery (in fact, it included slave states). The Confederacy fought for slavery, the Union fought for, well, union.