They are selecting soft, no gun zones as targets. Tactical units will arrive, but only after the shooter has killed a few or many, depending on how “well conceived” their plan was and how “skilled” they are with their weapons.
Airports, courts, police stations are no gun zones for civilians but because of security, they are hardened targets.
I really don’t believe that most of them are deterred by the thought of being killed. The really horrific massacres are pre-meditated and carefully selected by individuals with decent training and/or experience in firearms.
It doesn’t matter. Working alone a Seal team six operator could not clear a building full of people determined to take him down. You can only look, move and shoot in one direction at a time. This leaves all other angles open for attack. One man even with a gun is no match for a crowd of determined people