loose cannon 
noun
An unpredictable or uncontrolled person who is liable to cause unintentional damage: most of the cop heroes in contemporary films are loners and loose cannons...
- The establishment feared him as passionate and unpredictable - a loose cannon.
- She'd changed quite a bit since the days of her being classified as a loose cannon and an unpredictable quality on the field.
- Then there is the loose cannon of the Juventus side, the unpredictable genius who gives them an ability to score from nothing, a player capable of becoming the best in the world, if only he can curb his dark side.