Maybe the average person can just do risk assessment? "You might be the victim of a mass shooting" is significantly below the chances of "You might get struck by lightning." Which isn't an argument in favor of gun control or not carrying, but it does rather illustrate why your argument is not convincing.
Then that person isn't doing a complete risk assessment.
One of the tenets of most good risk assessment systems is the idea that you attempt to mitigate for a hazard based on risk if :
- There is a mitigation available
- The risk level of the hazard is high enough
and
- The cost of mitigating the risk is favorable to the potential cost of the hazard occurring.
Risk is a function of likelihood vs. severity. A mass shooting is really really low on the likelihood scale, but enormously high on severity. The residual risk therefore would be "high", which warrants control
The cost of mitigating this hazard is a decent carry gun, some training, some ammo. Pretty low.