Quote Originally Posted by Procrustes
I find that when I capture the King the ransom is often so high that it is refused and you kill him anyways. You may get away with a king's ransom once, but if you've been at war with the faction for a while I doubt it.
A faction will always pay the King's ransom if they don't have a heir to take his place (or if you have captured that heir too). Katank has based a Turkish strategy on isolating and ransoming the Egyptian Sultan several times by taking Tripoli.
But a ransomed King gains a 'captured vice' which lowers command but seriously increases morale, so be aware: he and his men will be less likely to rout next time.

About killing the general but leaving the troops: what you don't realize is that killing prisoners was frowned upon in the middle ages just as much as it is today. Prisoners were the property of their capturer, to whom they would swear that they would not escape, and in exchange their capturer would spare their lives and randsom them. Killing them was not only breach of oath, but stealing from your men as well. Especially the general, since he was worth most.