Defending your homeland from an attack by a powerful slave owning nation like the USA is not evil. It is the most honorable thing you can do.
And the US Constitution said nothing on the subject. The Declaration of Independence (which is the mother of the US Constitution) says all that needs to be said on the subject.
According to my history book, the South fired on Ft Sumpter before the North invaded the South.
I also do not believe that the South ever requested to negotiate terms of their secession with the Congress before acting.
The South seceded unilaterally as Buchanan’s term ended and before Lincoln was even sworn into office. Had Lincoln done anything other than what he did to try and keep the Union together, he would have been unfaithful to the Oath of Office that he took.