If the problem is being able to get a gun, then presumably these incidents have not just happened throughout US history, but were more common in the past when anyone could just buy a gun any time anywhere.
That doesn't sound ideal at all. The police are certainly the last people I would trust with deadly weapons.
They argue for having them for self-defence here too. I'd hazard that it would also be true that many in the US can say they don't know a single person that owns a gun. In South Africa they've made getting a license difficult and drawn out. This may be due to incompetence, inadequate staff or a government conspiracy (as some like to believe). South Africa has plenty of illegal guns in circulation though, and it wouldn't be hard for that to remain the case even if personal gun ownership were illegal. I doubt the US could keep guns out of the hands of criminals or in fact anyone who really wanted one. They can't even manage this in the UK.
Training and regular re-testing should be mandatory, no doubt. And definitely many times a gun will do you no good. But that doesn't mean they're useless. The next step after banning guns is to start getting really silly, as has happened in the UK.
It's really not that hard.
What's quite curious is that quite often those opposed to gun regulation are themselves thoroughly opposed to personal freedom on most other fronts, authoritarian even.
The 2nd amendment has however not prevented gun regulation. It only stops an outright ban. Now many will argue that the original justification for the 2nd amendment makes no sense in the modern day, that there is no possible way the citizenry could take on the might of the state with force. But the effectiveness of numerous small, under-equipped armies proves this not to be the case. It also wrongly assumes the army and police to be entirely free of members who would side with the people to bring down the government. Realistically anyone really attempting to violently overthrow the US is unlikely of course, but since the real problem isn't guns there's not much point in outlawing them. And it wouldn't really make them unavailable.