Ok, that's a list to work off. Problem is you are stilllaying the problem at hand on gun owners, plural. Instead of a single gun owner. I have owned several guns for my entire adult life, and not once have I ever even entertained the thought of using said guns on any part of the populace.
As for making guns harder to get, ok, I'm actually not opposed to that. I actually think there is a list of things we should make harder to get:
Guns
Drivers Licenses
Prescription Meds
and such
I actually agree that background checks should be harder for gun purchases.
But the opposite is also true, gun free zones don't work. In the military we are taught to make ourselves a hard target, because soft targets will more than likely get attacked. This guys manifesto even states something similar, that hard targets should be avoided because its too risky.
When was the last time a gun convention got shot up? Just making a point. Schools should have armed security guards. Places of public interest should have a higher number of armed security within a minute or two response time.
But I disagree with the narrative that gun owners are the problem, I even disagree that guns are the problem.
The problem, as the OP started thsi discussion for, is the underlying issue, hatred, racism, etc. This kid had a real problem. For all we know harsher background checks would probably not have prevented this individual from obtaining a gun. Nor would higher taxes, etc.
How do we deal with those issues tbat cause a person to use any tool necessary to inflict harm upon others?
Don't feed into their frenzy. Terrorist crave attention, its why most will claim responsibility for attacks after they happen. They crave legitimacy. When people stop living their lives because some terrorist did something, terrorists win.
