how and when you know it’s a necessity

I did the math actually. And it seems like mass surveillance will only be justified if homicide rates are higher than 20% ( if 1 out of 5 people die in murder ). And only if surveillance actually stops all the crime ( which it doesn’t ) and only if there is nothing less problematic that could be used instead ( which there are plenty techniques, like normal regular investigation, where you ask people around on their own terms ). Basically the math says it isn’t justified by an apocalyptic margin.