Interesting commentary. When the Taliban kills prisoners, the rest are supposed to be demoralized and leave the country. However, when opposing forces eliminate the Taliban prisoners, then they will fight to the death. Interesting commentary.
Pretty simple concept. If you bother to study the region, the Taliban do not have a very firm grip on power. The fact that a commander has already defected is a very interesting piece of intel. But if you kill the guy, no other commanders are gonna defect. It's a lot easier to defeat an army if you help along any inherent tendencies for it to self-destruct.
There is a difference between "Defectors" and "prisoners". Prisoners are those that have given up, generally in the face of attack, and are taken into custody. Defectors are ones that have changed sides, generally not under attack. While Defectors may not be particularly trustworthy, they are generally a desirable thing.