Honestly I would prefer someone with a CCW in basically every mass shooting incident. In a shooting, anything you can do to distract the shooter saves lives. I hate to say it, but putting rounds down range (within your ability) in the direction of the shooter, even without a 100% clear shot, will save lives. A mass shooter isn't looking for people coming after them, so you will have the jump on them instead of vise-versa like you assume, unless they singled you out as the first victim. If you are truly about saving the most lives, harming a bystander yourself is the least of your concerns as long as you are effective at making the shooter distracted, take cover, wounded, etc. The shooter isn't playing by the rules, I would be okay with a trained civilian doing something about it even if it isn't 100% clear-cut, close range, 1v1, open shot.