Al, you said, "Just an unhinged individual whom a caring society would have sorted out, and whom the authorities had been alerted about - but decided to do nothing for."

I explained why they had not been sorted out. There are just too many of them.

If that was not the problem, another problem would be that they do not want to be sorted out. They would have to be taken forcibly and remember that they are not known to have broken any laws. Happy with that?

Jim,
"The authorities are aware of 23 000 such "unhinged" individuals."
And how many arrests?

They can be arrested if there is evidence that they are planning an attack, and a number of attacks have been foiled and convictions made.

The Westminster killer, the Manchester bomber, and two of the London Bridge killers were known to the authorities, along with 23 000 others, but were guilty of no known crime before they struck.