How many times have you heard about this type of thing happening? I'm not saying that it doesn't (in reality it probably does a few times a year), but the inconvenience of having waiting periods for the firearms that are sold and not used in spur-of-the-moment shootings far outweighs the few lives that may or may not even be saved. Keep in mind that your scenario above can be equally applied to using other instruments:

Counter-points:
Two guys get in a fight, tempers flare. Guy #1 walks to his car, waits for guy #2 to walk down the street. Guy #1 runs guy #2 down and kills him.

Two guys get in a fight, tempers flare. Guy #1 walks down to the local sporting goods store and buys a hunting knife, baseball bat, whatever. Guy #1 stabs guy #2 (or beats him to a pulp, etc.).

The same can be said of just about anything, right? A large wrench, a hammer, a nail gun, an attack dog, an icepick, an archery set-- well, I think you can probably see my point. I haven't heard anyone ever argue about having 10 day waiting periods for getting any of these in the name of safety/having a cooling off period/etc.

I think that having 10 day waiting periods really comes down to a matter of simple politics and nothing more. Liberal politicians want to give the appearance of "being tough on crime" (whatever that means, since only law abiding citizens will obey a 10 day waiting period in the first place). Being in favor of waiting periods makes for good sound bites, especially in liberal states.

Let's state the real purpose behind waiting periods, then. Since I've just shown that different instruments can be used to accomplish the same end in your given scenario, and since there are no waiting periods to purchases these other instruments, handgun purchase waiting periods really aren't all about safety. (If safety was the main concern the measure would cover other things like archery sets, hunting knives, etc.) Since only law abiding citizens will go through the hassle and wait to get their purchase, it's not about keeping guns out of the hands of criminals.

Waiting periods reduce the "convenience factor" and create extra hurdles that gun shops and gun purchasers must go through, thus potentially resulting in fewer weapons purchases and definitely resulting in frustration on the part of gun shop owners and gun purchasers-- frustration that the government is meddling in what should be a private affair, putting its nose once again in a place where it doesn't belong. Waiting periods are little more than a political stunt, used to give the impression that something is being done to make people safer.

[Edit]: Also, just wanted to let you know a little about me... I live in Northern Virginia. When I originally moved to the area (about 10 years ago), I looked at Maryland as a place to live (wanted to avoid those darn car taxes ). Then I looked at Maryland's gun laws, and also saw that it was a rather liberal state. A few years ago, when I was moving, I looked at Maryland as a possibility again. Then I realized that getting a CCW there would be next to impossible, and saw that some things (like large capacity magazines) were also illegal. Scratched it off my list once again. My point: anti-gun sentiment in states like Maryland can actually hurt the state in the long run, as people decide to locate and shop elsewhere because they feel they aren't welcome. Even though I haven't gotten my CCW yet here in VA, knowing that I can if I want to means something to me. It represents a freedom that I have that many Marylanders will unfortunately not be able to experience.