I would just say that the key part to include is that the North knew slavery needed to die on the vine and was uninterested in helping the South preserve it, specifically by opposing the addition of new slave states, or at least abandoning the notion that the two should be intentionally kept in balance.

So nominally, yeah, few with any influence were proposing emancipation, and to be clear almost every white person in the country was super racist by modern standards, but slavery was doomed over the medium- to long-term. The South could see that the writing was on the wall, so they decided it was time to shoot their shot to preserve slavery, in a form particularly at odds with the world around it by the way, for as long as possible, and secession was the only viable path for them. No other issue of the day would have driven any significant region of the country to secede, though ironically if it had, no other issue would have given the opponents the moral high-ground like slavery did.