On top of all that, crowds disperse for cover when someone starts shooting. Mass shooters seldom go to ground or seek cover because they need to move around to pursue victims. They also obviously don't want anyone within arms-reach of them because they will be tackled and disarmed. So, no, they generally aren't in positions to create cross-fire problems if cops, security, or armed citizens attempt to bring them down after they start shooting. Never mind the fact that most self-defense weapons and ammunition are chosen for stopping power, where a bullet lodged into the target and transfers all it's momentum into the assailant rather than rips right through them.
It looks like all Ms. Marsden learned about guns she learned from Hollywood.