But if those 21 people or 315 people are also armed. It would take all of about 30 seconds for him to be dropped.
Look at the Virginia Tech situation, if they security guards were armed they could have stopped him, but no one had any guns on school campus so he was free to just keep going.
Edit: I fail at quoting
you are right the virginia tech tragedy could have been much less if they were armed you are probably right. Also, they could check people for guns, If he lives in the dorms Guns are not allowed, he would have never had a gun in the first place.