Firing at Ft. Sumter was indeed a stupid mistake. One high ranking confederate begged the leadership not to do it. He told them it would completely change the current state of grace they had with the friendly Northern states, and it did.
Lincoln had taken NO action against the South until that stupid incident. The South might have been able to peacefully separate but there were too many hot headed Southerners who wanted war because they thought the vaunted Confederate fighting spirit would save the day.
There is a pretty compelling argument that Lincoln deliberately coaxed them in to that attack because he needed them to attack to justify what he wanted to do to stop them from seceding. It would appear that he laid out a trap, and they stupidly blundered into it.
In any case, the response was massively disproportionate.
Sherman served as the superintendent of what would become LSU up until right before the war. He warned, begged and implored his compatriots in the south against embarking upon the route they did. Beauregard had a high enough regard for Sherman that he tried to recruit him to the confederate cause, which Sherman declined. Conf. General Joseph Johnston, Sherman's opposite in GA and the Carolinas held Sherman in high enough regard he served as pall bearer at Sherman's funeral, and per Wikipedia, "It was a bitterly cold day and a friend of Johnston, fearing that the general might become ill, asked him to put on his hat. Johnston famously replied: "If I were in [Sherman's] place, and he were standing in mine, he would not put on his hat." Johnston did catch a serious cold and died one month later of pneumonia."
"Unconditional surrender" was only "disproportionate" in the same sense as the US response to Pearl Harbor and Hitler's declaration of war were also "disproportionate".
Sorry, but the correct response to a declaration of war on the United States is "unconditional surrender".
However, destruction in the Confederate south was massively less than Axis powers in WWII.
As for Lincoln's alleged "trap", first, he clearly announced in his Inaugural Address that the Confederacy could not have a war unless they themselves started it.
So Jefferson Davis was fully notified.
Second, Lincoln offered to abandon Fort Sumter, for the right price, and his price was a pledge by Virginia not to secede.
A fort for a state was a good deal, in Lincoln's mind.
Of course, Virginians could make no such pledge, because they themselves needed war to start at Fort Sumter, in order to meet the conditions of their Constitution ratification statement.
War allowed them to claim "oppression" and therefore secede.
Lincoln, realizing he could get no promises from Virginians, left the issue of war and peace in Jefferson Davis' hands, and Davis decided on war.