The possibility of being shot by police arriving on the scene is not that great, unless there was an extended gun battle.
As they say "when seconds count the police are only minutes away".
I wonder what the exact response times were in the major mass shooting sprees?
Response times to a 911 call are just one factor, how long it takes for someone to make that 911 call is another factor.
It took quite some time for the Colombine duo to track and shoot that many students. If a single person at that school and in the vacinity of the shooting had being carrying he very well might have ended that spree in seconds. Long before the first 911 call went in, and certainly long before the first police showed up.
If I'm not mistaken that seems to be common in such incidents.
Once the shooter is down theirs no need to be standing around with your gun in your hand.
Like Tinner I live in an area where murders and even mass killings are not at all uncommon. Its something you learn to live with.