I agree with what you say, but would like to expand on it a little.
Prior to around 1850 there was no such thing as a North vs. South dichotomy in American politics. Since at least the War of 1812 the country had been politically split into three sections, East (what we now call the Northeast), West (the free states from Ohio to Minnesota and Iowa), and South (the slave states). Some states, like MO and KY sort of vacillated between West and South.
All this time the country had been generally dominated by an alliance between South and West, both of which were heavily agricultural and thus economically aligned in interest against an increasingly industrial and financial East. The notion that in 1850 MS was agricultural and IA was industrial is just ludicrous.
For most of this time slavery was a back issue that most Americans just wanted to go away. Abolitionists were assaulted and sometimes murdered in the West and even in the East. They were (accurately) viewed as disturbers of the national peace.
All this was based on the notion that the expansion of slavery had been settled by the Missouri Compromise in 1820. North of the southern boundary of MO would be free territory.
The southern extremists started pushing for the right to expand slavery into all the territories, with the Dred Scott ruling and the Nebraska Acts.
Men of the West were perfectly happy to have blacks enslaved in the South, but did not want to live next to them, or to free blacks for that matter, so in self-defense they turned against the South’s drive for expansion beyond previously accepted limits.
Meanwhile southerners, accurately or not, felt more and more at risk and they saw as mere self-defense what the East and West saw as aggression.
Which meant throughout the 1850s both sides saw themselves as merely defending themselves against attack by the others. The very natural and human response was to line up with others who felt the same, creating the North vs. South split that finally showed up in the 1860 election.
I see even most of the fire-eaters as thinking of themselves as in self-defense mode, not attack mode. They believed, probably accurately, that the only chance to preserve the institution of slavery in the long run was for the South to be its own country. Hence their maneuvering to create conditions under which the Union would fracture.