Well he was completely outgunned, it's quite possible that if he didn't become a casualty himself, he might easily have missed and hit innocent people.
What would you have done in his position?
90-seconds after hearing the shooting starting, you get to the building, and hear semiautomatic rifle fire.
Your only firearm is a pistol that you trained with but have never used in action, and the accurate range is maybe a few scores of feet.
You do know that the rifle will significantly outrange you, and that the shooter has prepared enough to set off the fire alarm.
You won't know any more than that.
If you stay outside, you might be able to stop the shooter leaving the building. If you go in, you might shoot innocent people, you might get shot, you might struggle to find anyone.
As has been said upthread, going in with a pistol is basically an action movie.
At Colombine, the armed guard did exchange shots with the shooters but failed to hit them.