If it was just about slavery, the North could have let the South go. The South’s declarations were about slavery. The North went to war about money.
Secession was for slavery as the Declarations say.
The south went to war over The Particular Institution. The north went to war over security. People who had been partners, neighbors, and even kin initiated hostilities against us and declared themselves our enemies. They breached the constitution, the law, and the peace. They stole everything that wasn’t nailed down and promised to steal more.
The set themselves apart from the rest of the union and said “We aim to kill you”.
The reaction from the north was perfectly appropriate, if perhaps understated and tentative. Lincoln approached it like a spat, hoping that cooler heads would prevail. It took him awhile to realize that there would be to sensible talk coming from the southron fire-eaters.
Lincoln did what he could and what he had to do to preserve the union - the whole union.