Constitutionally speaking, he couldn't. How was he going to try to keep it from spreading? You are aware that efforts in halting the spread of slave labor had little to do with humanitarian reasons, correct?
The southern states seceded/rebelled because they saw slavery coming to end, and they didn’t want to end it.
This conflicts with your statement above. How was it going to end? Who was going to end it?
They thought they could forestall it by secession. They saw it ending in a U.S. which they were part of. So they thought they would try to creat a separate country and keep it going for as long as possible.