Being on the outside of a building and hearing gunfire inside might be a call to arms for some, but being on the outside means you are ignorant of exactly what is going on in there.
Running back to your vehicle and retreiving an AR then rushing in cold might put you in the middle of a shoot out between a Narc stake out squad and a Zeta hit team or something equally unexpected, robbery in progress, etc. At the very least you'd impede the exit of victims trying to escape.
Should you make it inside you'll be sillhouetted in the doorway rifle in hand and obviously an immediate threat to the shooter. If he spots you first, and if shooting at people running for the exit he almost certainly would, theres no element of suprise on your side. He wouldn't even have to shift his aim by much.
Then theres the CCW who figures disgression is the better part of valour, perhaps trying to get his family out of the way. He and his are fleeing a man who's shooting everyone with a Bushmaster and here comes another man with a AR blocking the exit. Possible bad news there.
If already on the scene theres no excuse for not doing something. Going in blind and ignorant seldom works out well for anyone.