With many of these shooters there are signs that aren't acted upon so it's not always a matter of just people who no one knew had a problem.
Again I am not against hardening schools, to me it is just a measure that would have limited effectiveness
I don't see why saying they are troubled instead of evil provides any sort of excuse. Blaming evil adds nothing to the discussion, we can't fight evil. However when we see these people as troubled now that gives us avenues to work on the problems. Bullying, problems at home, depression, substance abuse, etc. All things we can address. Calling them evil just demonizes them, makes us feel better in judging them, it doesn't do anything else. I would say we can call their acts evil but when it comes to the individual we need to see that something is wrong inside and sometimes it can be fixed before these things happen.
Just law firearm safety, there needs to be multiple measures in place in case 1 or 2 or 3 fail.
Madison Stone Douglas, the shooter "was left behind". He begged to return back to SPED after showing a huge decline in academics after being transferred to regular classes. The system failed him. IMO, this could have prevented the situation, because he then stopped coming to school. (downward spiral)
Columbine, the shooters were bullied and on anti depressants (which means mental help sought). So only real failure here would be if the bullying was reported and not stopped.
Sandy Hook, the shooter killed his mom and stole her guns. No real system failure.
MIT shooter, college level, IDK much details on the why. But this was the largest active shooting deaths until the Vegas shooting. 2 pistols used, 9mm and 22LR.
Ulvalde, no system failure, unless his bullying was reported and nothing was done. He had no mental health professional help sought and no felonies.
So as you can see with the school shootings listed above, sometimes there is a system failure and sometimes there isn't. So evil needs to be met ASAP. Which is why giving staff the choice to be armed IMO is the best and least cost effective thing to try. There are groups out there who will train teachers for free. Like the Ohio situation after MSD. They trained over 300 teachers in a month, all free. I can guarantee if there was a magic machine that could go back in time and give the teachers who died an option to either have a gun, gun free zone sign, or a phone when the active shooter shows up, all of them would choose a gun.