Stopping power of the round fired and the accuracy of the gun determines it's lethality not rate of fire it doesn't matter how many shots per minute you can fire if they can't stop the target or even hit it. If you need a rounds effective stopping power, there are ballistics formulas all over the place.
???? How do you figure, since the U.S. murder rate is up from when full autos were legal and readily accessible.
No, it's not because they are illegal, because as it has been pointed out, they are legal if you meet the proper requirements. People aren't killed by full auto weapons often because the guns are easily traceable, they are expensive so it isn't a weapon you'll just ditch somewhere, the rounds get expensive after a while. A cheap .38 will kill someone just fine, and a 20$ street gun isn't going to be missed when thrown into the river.