Agreed. However, the people we are referring to is the "foreign" (not Afghani) Al Qaeda fighters.
The Northern Alliance spares Afghani Taliban but has killed all the foreign Arab, Pakistani, Chechen, etc. Al Qaeda fanatics it can get it's hands on.
The Afghani Taliban will melt back into their villages after the war and not cause any trouble outside of Afghaniastan.
The foreign Al Qaeda fanatics, however, have shown that they are ready, willing and able to travel throughout the world to fight their jihad against America. Every one of them that is spared a glorious death in battle by a U.N. brokered surrender is a potential future terrorist suicide bomber. Those are the people who need to die in battle. Every last one of them.