We don't need to look at any of the Southern states reasons for leaving, because that had nothing to do with why troops were sent to invade their states.
The only positions we need to concern ourselves with is why Union troops were invading those states. Lincoln controlled the bloodshed. No one else controlled the bloodshed but Lincoln.
He could order men to fight, or he could order them to stop. The Southern states had no control over the violence, because it was being visited upon them, not the other way around.
What were the Union reasons for invading? That is the only question that matters.
No president worth his salt would not come back at such an aggression with force. Write it on your hand so that you won't forget.
That's what governments do. That's what governors do when the National Guard or the state police get shot at. Thinking that they wouldn't is crazy talk.
So now the question is why the original attack. Find out what was behind that and you may get closer to the reason for the war.
In any event you don't get to decide on your own what "the only question that matters is" -- nor does anybody else.