a person who does some harm to another person because that person has done some wrong to him/her earlier, usually by punishing the wrong-doer severely and getting satisfaction by doing so;
a person who takes vengeance; a revenge-seeking person; a retaliator,
“The avenger confessed that he killed the underworld boss in order to avenge his brother’s death”, i.e. the man (the avenger) killed a bad guy (the underworld boss) because the bad guy had killed the avenger’s brother some time ago.
Robin Hood a romantic English folk hero avenger. (from Answers.com/WikiAnswers)
‘The Avengers’ is a group of Marvel Comics super heroes appeared in several comic books and now being made into a movie.