Loose cannon
WordNet
noun
(1) A person who is expected to perform a particular task but who is out of control and dangerous
WiktionaryText
Noun
- a cannon that breaks loose during battle or a storm and causes serious damage to the ship and its crew
- an uncontrolled or unpredictable person who causes damage to his own faction, political party etc.
- Ron Paul is considered a loose cannon in the Republican Party due to his opposition to the Iraq War and libertarian views.