We all know that the Declaration of Independence (which underlies the U.S. Constitution) speaks of certain "inalienable" rights coming from the "Creator." Well...whether you believe in God or not, the point is that certain rights PRECEDE government. For example, the right to free speech, free practice of religion, and freedom to assemble ARE NOT GRANTED BY GOVERNMENT. They PRECEDE Government, and the purpose of Government is to create a society where these inalienable right can be preserved and exercised.
Among the "inalienable" rights is the right to protect oneself and one's family, and to fight oppression - mainly by Government. It is in this spirit that the Second Amendment was articulated. In other words, the right to protect oneself - with "Arms," if necessary - is not a right that is granted by Government. It PRECEDES Government, and Government is obliged to protect that right.
So Government does not have the power to restrict the right to bear Arms without due process and good cause. Just as Government may not SILENCE someone based on the content of their speech without due process and good cause. Hence, the right to bear Arms is taken from many convicted felons, sometimes permanently and sometimes for a period of time.
But Government does not have the power, under our Constitution, to wholesale remove the right to bear Arms from large swaths of the population for trivial reasons. Or even logical reasons.
So if someone is known to have a "hot temper," or is known to get involved in bar fights, or has been heard threatening people with physical harm, or even is "a little bit crazy," these are not sufficient to take away the right to bear Arms without a very specific statute that has passed Constitutional muster, and Legal Process as applied to every individual affected by the law.
Every time we have a "mass shooting," the politicos and journalists and general do-gooders send up a hue and cry, "Why don't we PASS A LAW???" "Why don't we DO SOMETHING???"
But as we get into the nuts and bolts of passing a law or "doing something," we run into a conundrum. We can't establish a public policy that - let's say - would have prevented the most recent shooting (whatever the particulars) without impacting the rights of thousands or millions of other innocent people who pose no real measurable threat to anyone. That's why the magical "law" that would prevent these things never comes into existence.
You can look at other countries and see that they don't have the same issues with gun violence, but you can't just wish away the U.S. Constitution. Those countries were founded on different principles. It is what it is, and the laws that may be effective elsewhere would never pass Constitutional muster here. So we have to live with the fact that in a country of 330 million or more people, there will always be some crazy bastard doing what crazy people do, and there isn't much we can do to prevent it. Just consider that the number of people killed in "mass shootings" is a tiny, tiny fraction of all felonious deaths in the U.S., and we should focus our attention on the problems that CAN be solved rather than the ones that can't.