Don't waste a single opportunity. Don't live life thinking, "I'll tell them next time", "I'll visit them tomorrow"... Next time and/or tomorrow may be too late.
Not a revenge-oriented person, but really. No need to even avenge ourselves. (1) God is our avenger (2) Bitterness is poison (3) Some people just don’t deserve that place in your life. Hate and bitterness take too much energy that could be put.