For intelligence to be effective, the government will need to collect personal information, like bank transactions, emails, phone records, and more, without the citizen in question knowing this. However, democracy works on the assumption that each citizen has a private sphere, separate from the public sphere, of private information, thoughts and opinions, and that the citizen decides who to let into that sphere. Put differently, the citizen has control over when to release private information, and when not. Investigating them without their knowledge means taking away that control, and that is a violation of the right to privacy. Police investigations, on the other hand, are legitimate and not invasive of democracy - the police are generally obliged to inform a citizen when he or she is subject to a criminal investigation, and can generally only investigate a citizen without their knowledge after seeking specific permission from the judiciary, not just by a minister signing off a whole batch of requests as it’s done with domestic intelligence.