The 5.7x28mm round used by the Fort Hood shooter is no more powerful than a .22 rimfire magnum. It's crap. Why? The 40 grain 5.7 is blazing down range at 1750 fps from a pistol. The 40 grain rimfire mag is moving at more like 1200-1300 fps from the same barrel length. The 5.7 has 80% more muzzle energy than the .22 mag. That's a lot people. Now maybe they'll both through and through with similar terminal performance, but you can't just dismiss that level of difference.
Is the 5.7x28 round a copkiller? No. There are no instances of it being used to actually kill cops. For that matter the civilian legal ammo isn't even particularly dangerous to people in common police body armor. At least no more dangerous than 7.62 Tokarev.