Arguable. If someone said "It's a terrible and undemocratic idea but it isn't slavery" that would be perfectly acceptable to me.
And if I remember correctly,no one was killed in the firing on the revenue fort.
Do you think the people firing the cannons thought they were not going to kill anyone? If they did think that wouldn't they be the biggest fools under the sun? Note that the main reason there were no casualties is because the commander only let his troops fire back from the guns that were safe. Men mounting a full defense would have been exposed to heavy fire and there would have been casualties.
I think it was clear the South's policy and desire was to be let go their own way.
Yes, they started a shooting war to go their own way because they thought a non-abolitionist might impose abolition. That's not a justification for killing anyone.
The whole "preserve the union" reminds me of Old World history that hails King Whomever for "uniting a country" when what happened is that he killed or caused to be killed anyone who resisted his rule.
There were legitimate reasons to oppose secession, and comparing it to some despotic power grab is infantile.