I think there is too much emphasis put on caliber. One who is mentally committed to extreme violence should have a good go at finality regardless of the tool at hand.
That's true to a certain extent; a .22LR can be just as deadly as anything else on the market, but shot placement becomes very critical as the energy imparted decreases.
From what I've read, 9x19mm beat out the .45ACP in U.S. military testing for several reasons. Perhaps the most important is that our European NATO allies were already standardizing on 9mm and we wanted to simplify logistics in preparation for the cold war going hot. Of course, for civilian use that isn't important, but (at least in peacetime) surplus ammo is a lot cheaper to shoot.
The factor that was most controversial was force delivery; since Newton's Second Law states that force = mass x acceleration, force is directly proportional to a bullet's mass, but is proportional to its velocity squared. So, lighter, faster bullets hit harder, or that's the theory. Again, from what I've read, not personal experience, the much greater penetration of 9mm FMJ often causes it to go through a person, so much of that force is still in the round as it exits the body. The .45ACP on the other hand, imparts more, if not all, of its force and being a large, heavy round, is especially good at transferring momentum.
Most of the newer popular rounds try to bridge the gap by being larger and heavier than the 9mm for increased momentum transfer, but they have higher velocities, flatter trajectories, and better penetration than the .45. .40 S&W and 9mm seem to be the most popular calibers among police departments in my area; my guess is that 9mm's popularity is mostly due to cheaper ammunition.
