For one thing, we can get better intelligence from a live enemy than a dead one. It's also a good policy to encourage other terrorists or spies to surrender, rather than fight to the death. Lastly, we're supposed to be the good guys, so we just can't go around blowing away the bad guys just 'cause we feel like it -- it may be emotionally satisfying to some, but it's just not professional.
i get your point and certainly agree but i was only addressing the Geneva Conventions rules aspect. :)