It sounds crazy 150 years later, being willing to put the Union asunder just to keep the right own people as property.
Let me correct this false notion. The Union already recognized the right to own people as property. Remaining Unified, the Union would have continued to recognize the right to own people as property.
Therefore, the right to own people as property was not why the Union was split.
If you doubt this, look up the Corwin Amendment, which passed both the House and Senate with mostly Northern state majorities. This amendment would have made slavery nearly permanent and Lincoln urged it's passage in his first inaugural address.
The dispute was not over slavery, it was over whether the Southern states commerce and taxation would be controlled by Washington DC, or whether they would govern themselves.