When the only tool you have is a hammer, you try to make all your problems look like nails.
They couldn't tell the truth; That they wanted the South to keep paying a disproportionate share of the bills; That they wanted to keep making those millions of dollars from the labor of slaves;
The truth wouldn't work, because regardless of what the businessmen and the governing rulers were, the ordinary people were quite moral and they would not be induced to attack others simply because they were losing jobs and trade to them.
No, the ordinary public had to be induced into this, and the truth would not motivate them to war.
Thus the hyperbole and propaganda.
Hyperbole & propaganda comes 100% from you Lost Causers, no Unionist Republican is sitting around concocting nonsense when actual history doesn't serve.
Lincoln well understood that war over money could not happen, indeed should not happen.
But war over rebellion, insurrection, "domestic violence", invasion and treason were much different matters.
So Lincoln first warned secessionists, and then waited.
He did not have long to wait.