Ideally, they should have let them secede. Slavery was in its last throes anyway. Yes, it would have lasted longer and hundreds of thousands would have remained enslaved. But hundreds of thousands died, more were disabled, and a greater evil in the form of an ever centralizing and geographically larger State was born as a result. Which happened to classify black people as inferior for a full century after that anyway.

When judging participants it's helpful to separate the Secession from the War. The Secession was an act to preserve the status quo of slavery by asserting what those states thought to be their self determining rights. The War was an act to prevent secession based on the notion that states, having moved from colonies of the crown to members of the union, had never been self sovereign and therefore did not have the right to leave. As war broke out the Union found it didn't have the military support that it expected and needed something to galvanize support. As Grant would later say, "when the war broke out the army dissolved. We had no army – then we had to organize one." This, after war had begun and lines had been drawn, is when the states purpose became emancipation. The average soldier didn't care about preventing secession, but they could be stoked by a crusade to end slavery. With this framing you could say that to the soldiers the War was in fact about slavery, but if we want to judge leadership and governance, slavery was profit to the Confederacy and a tool for the Union. As Reddit would say: Everyone Sucks Here.

The South wasn't a bunch of innocents, for sure, but the question we need to raise is, did he need to wage war? if you think that he did, then I would argue you haven't studied history enough. Why did chattel slavery end everywhere else before and after the Civil War without a war?

I believe revoking the Fugitive Slave Act and letting the South secede would have ended slavery in the same way it did in Brazil, through economic means and without millions of deaths.