I think it would have been impossible for the Republican party, founded to oppose slavery, to allow states to secede so they could continue to practice slavery

I think it would have been impossible for the Republican party, founded to oppose slavery, to allow states to secede so they could continue to practice slavery on independent soil. People absolutely were willing to die over this: that's the whole point about bleeding Kansas. In general, more people have died in wars have been fought over much less, either morally or materially.