This all starts with and activist Supreme Court supporting the southern states in a ruling on "slaves" still being a slave even outside of slave states and that the federal must impose slave property rights in all states and federal territories ...
up till then the great compromise is have been working to some degree and slavery was going to slowly wither away and I as intended by both sides..
There were lots of Southerners that did not support slavery and wanted to see the institution dry up in a controlled manner... think Lee himself
but then the damn activist Supreme Court kick the anthill and feeds the southern slavery hotheads hardliners wants.
So then you have south then wanting the federal to impose the South's "rights" on the norther states and peoples and the free territories....else they will leave the Union...
And bound by honor... you even have the non slave supporting Southerners going along to support their state if they leave the Union....
The problem is that slavery was NOT withering away in the late 1850s. It was by all evidence thriving, with slave prices hitting their absolute highest point in 1860.
The price of any capital investment is based on the general perception of its future profitability. Therefore, by definition, in 1860 slave buyers did not expect slavery to begin withering away.
That is simply a projection into the past of post-war attitudes.