In the oldest daughter situation, you don't advocate lopping off her fist so she isn't a threat did you?
The 2A was set up as a personal right to self defense, that self defense being against enemies both foreign and domestic. Yes, I stole that from the military officer oath of office, but the statement stands and hold water under the SCOTUS rulings, that our personal defense is important and upheld by 2A.
SO, why should we ruin someone's ability to defend themself by limiting the ability to get a gun when there is no reason not to? We all agree those who lie on the form 4473 are breaking the law, and those situations need to be ignored in this mental exercise. But lets remember a lot of people move out of their parents house around 18yo, not as many as before but that seems to the time we deem a lot of kids adults, they graduate HS and can vote, so they are adults for legal purposes.
Lets say your daughter moves into an apartment by herself and can not legally get a gun for self defense (due to the new laws that will be put in place due to this shooting) and she is violated by some perp....whats more important, the right to self defense or the fractionally decreased chance that a kid hellbent on killing might have to wait a little longer to work out their plan than they thought? Don't forget, most of these killers have been planning these scenarios for months if not years, so what another couple years and maybe that would further their plan and give them more ideas for worse things such as bombs or explosive filled vehicles or whatever...these are sick people and trying to think rationally about them is futile.
Consequently, no more laws are going to prevent these situations. That is a lose lose situation. The only way to prevent these situations is to find these perps before they hit, or stop them in their tracks (somehow) after they start.
Trying to stop them before they start is a tricky thing, and usually looking in hindsight there are lots of warnings but seeing these in forward speed they don't usually raise to the level of credible threat.
So how do we stop them in the act? Not incredibly difficult but it will be expensive. I have suggested two easy fixes that would be able to be implemented but the cost will turn any lawmaker away...ballistic glass and ballistic closets. There can be many more options, but those two would stop most of the mayhem in these situations. Not all. There is No fool proof method to remove all threat of crazy people in the US.