dangerous person; security risk
A person who poses a physical, criminal, or security threat. Often used in law enforcement, security, or warning contexts.
He is regarded as a dangerous person by the police.
A person on the security risk list was detained at the airport.
loose cannon
An unpredictable or uncontrollable person whose behavior could cause trouble, especially in an organization or team.
That new guy is a loose cannon; you never know what he'll do.