Julius Caesar's kidnapping was a financial enterprise.
Kidnapping for ransom was a tried and true way used by the criminal fraternity to raise money during the Roman hegemony, on land and on sea.
Our current kidnappers are using it for political ends. Thus, killing the victims makes perfect sense if the victims are going to turn around, once released, to try to kill their captors. If they have no interest in turning in undamaged goods for money, why should they leave troublesome, vengeful witnesses hanging around?
The OK Corral is not the model of law enforcement that Iraq needs to emulate.