OK, I agree that revenge must be at least a partial motive for killing the wounded. But you must admit, if your best friend had just gone to give a wounded combatant some water, just to have his head turned into a mass of goo by a grenade the Iraqi had hidden up his ass, you would be wary and angry.
The Marine's jobs are to stay alive and kill enemy combatants. If the wounded sometimes shoot back, then why not. As for bombing the entire country into the stone age (like thats a step backwards for that country), if everyone in the country were to become combatants, then yes we should, but since they are largely "innocent" civilians previously brutalized by a corrupt regime, we should at least attempt to preserve their lives.
Try to put yourself in our soldiers place. The last 3 Iraqis you have gone to give medical attention had grenades hidden under them. 5 out of 10 bodies lying on the last road you passed came to life to fire on you (remember the shotgun-stretcher video everyone?). Wouldn't you make sure the ****ers were dead?
While these numbers are probably exaggerated, you only have one life to live, might as well not take any chances.