Refers to someone who betrays their lord or country, often in a historical or dramatic context. Strongly negative connotation.
He was executed as a traitor who betrayed his lord.
In history books, that warlord is recorded as a rebel.
Compound of 逆 (gyaku, 'reverse; oppose') and 賊 (zoku, 'bandit; rebel'). The exact historical derivation is uncertain, but the combination conveys the idea of a rebel who acts against legitimate authority.