A quote from Oxford Advanced Learer's Dictionary of Current English:
avenge vt get or take vengeance for: ~ an insult; ~ oneself / be ~d on an enemy (for an injury, etc). He ~d his father’s death upon the murderer, punished the murderer.
My question is: Why does 'be avenged' (in the passive voice, though) mean 'get or take vengeance' (in the active voice)? Why is 'avenge' used in the passive voice?