There is a saying, which lawyers quote, along the lines that exceptional cases make for bad law.
Personally, I believe that a combination of the easy availability of firearms and mental illness is a pretty lethal cocktail. No one is going to take away an American's gun any more than you can separate a German from a fast car and the "right" to drive it as aggressively as he possibly can. These are regarded as "civil liberties".
I think you will find the wording of the law in most countries is something along the lines of, "You may intervene when there is reasonable cause to believe that..." Trying to find a one size fits all definition of "intervene" - let alone "reasonable cause" could keep the lawyers busy for a long time. In short, I do not know and I feel considerable sympathy for those, who are required to make those decisions - often at very short notice and with imperfect information.