“The deep South seceded before Lincoln was inaugurated, and the rebels fired on Fort Sumter three weeks after.”
Yes, and that was in direct response to the election of the tyrant. The south didn’t wait for him to take office; the writing was already on the wall.
Ultimately, the rallying cry of the north was to force those who no longer wanted anything to do with the union to stay a part of it.
Lincoln was a threat to what, exactly? Lincoln was no abolitionist. He WAS opposed to the extension of slavery and was an opponent of the fugitive slave act. I presume that you are a Lincoln man on those issues.
Lincoln also assumed, as did many other observers at the time, that slavery was a dying institution and that confining it within its historic boundaries would hasten that death. The slow eradication of slavery in the northern states was an instructive example. By 1860, Delaware -- still nominally a slave state -- was on the threshold of the disappearance of the peculiar institution. Maryland and Missouri were clearly headed in the same direction. Slavery would have ended with or without the war. Historians debate how soon; I lean towards earlier rather than later, but the institution's days were numbered in either case.
The deep-south slavery perpetualists saw the same dynamic at work, and feared it. This was the radicalism that drove secession, not northern abolitionism.