"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
We can interpret this in two ways:
1. Because the U.S. did not have an organized military established to defend it, a standby militia made up of armed free citizens was necessary. And therefor, so was the citizen's need/right to keep and bear arms.
2. Because free citizens need to maintain the right and the ability to control and even possibly overthrow their own governments to maintain their freedom, their right to keep and bear arms must not be infringed, particularly by their government.
The use of the phrase "a well regulated militia" would certainly imply the former interpretation, rather then the latter. However, we know that the founders did believe in the right of the people to control, and to even overthrow their governments, when necessary, to preserve their own freedom. At the time, this would have been an automatic possibility when the only military force the government had access to was a militia made up of free citizens. Now days, however, the government maintains it's own standing military, which we HOPE would act on behalf of the people in a clash with their own government, but that is not intended nor set up to do so.
So that now days, that second interpretation, even though probably nor originally intended, has a lot more import than it would have at the time it was written.
All this aside, however, it is in no one best interest to arm criminals, fools, idiots, drunks, dope fiends, rage-mongers, and the otherwise blind and/or insane. So clearly, regardless of how we interpret the Constitution, some intelligent regulation is both wise and necessary.