The death rate from handguns is only 20% depending of course on the bullet placement, but in general, the FBI stats show an 80% survival.
Understanding that fact, playing dead while a weirdo walks up to you and puts a couple of bullets in your brain with exceedingly high mortality makes the option of going for a tackle a much better statistical chance. I hope I never have to put these stats to the test, but I believe I would be moving either away or towards the shooter depending on the situation.
I was actually surprised that the Fort Hood soldiers did not mount an attack in some manner against the Islamic terrorist now on trial. Besides tackling, there are objects you can throw as a distraction/weapon to help with the attack. Doing nothing and just letting the weirdo walk up to you and kill you is just not an option in my mind.