FBI Director James Comey has stated publicly that Sen. Hillary Clinton and her staff were careless & negligent in their handling of information that was marked "Confidential" or that they knew or should have known was secret. He indicated that such persons would, typically, be stripped of their security clearance.
In that Hillary Clinton has demonstrated that she lacks judgement in handling sensitive and secret documents, it is clear that she should not be trusted with briefings on matters of national security.