Reputations go a long way.
If these S. Korean hostages are murdered by the Talibanis, then they can look forward to a South Korean expeditionary force dispatching itself to Afghanistan. The Chi-com supported VietCong feared Korean units for good reason during the VN war. They were downright inhumanly ruthless against the VC.
Killing even one of these hostages would be the biggest mistake the Taliban ever made.