No. What stops a mass murderer is either he/she runs out of targets and or ammunition, or someone shows up with a firearm and puts the shooter down. The first option isn't very attractive - it means the whack job has accomplished maximum casualties. The second option is far better, and the sooner the better. Rather than wait for law enforcement to arrive and screw up the courage to intervene - something that is demonstrably a minutes long process and apparently not a sure thing... Why not arm, or at least allow to be armed the people who are already there. The sooner one of these incidents is stopped, the more lives are saved. Why would anyone be against the one provably effective measure that could save lives? They must have some overriding agenda, something more important to them than saving lives.
They try very hard to cover up the actual timeline. But it looks like more than 10 to 15 minutes after the police are on scene before they take action. They wait for the swat guys, the armored truck, etc.
And they don't even let the rescue helicopters launch because there might be gunfire. Of course those helicopters are often flown by people who learned how to do so flying helicopters in and out of firefights in Vietnam, Iraq, or Afghanistan where they didn't wait for the gunfire to end before landing let along taking off.
And the EMTs stand around outside waiting for the all clear, even though some of them learned to, and have done the same job in Iraq or Afghanistan in the middle of a firefight.