It's a feeling , a very dark and evil thing , it's more like a monster grows inside of you , cased by bad things others do to you and not being able to take your right back from them ,, i mean they have caused you so much pain , left you with deep scars , then you see them happy with their lives not caring about what they have done,it's not fair is it !! they don't even bother themselves saying "Sorry" i swear it would've been enough ! Revenge is when you reach to the point that you want to hurt them as much as they hurted you ..