The Stasi is a good model for how quickly and badly this can go off the rails.
The way it's supposed to work is that you have some public event of importance: a crime, a person applies for a security clearance, or whatnot. Based on that event, the state then conducts and investigation to see if there are any data they can discover that might lead to a response.
The new way of working is that you collect all the data, on everyone, all the time. Then there's no more event. The only thing you do is pick out the person. Once you have the person, it's a simple data mining exercise (along with a lot of interrogation techniques) that takes care of the person. What's the old saying in communist countries? "You have a man. You have a problem. No more man, no more problem"
Functional societies focus on events. Dysfunctional societies focus on the people.