Is Revenge Sour or Sweet?

"Anybody ever asks you what the sweetest thing in life is-"said Lazzaro, "it´s revenge." (p. 139)

Revenge is a topic that is constantly seen in life. We even have it injected into our minds from all around us. Role models, books, movies, plays, stories of the past, they all show us revenge and teach it to us. sometimes it is seen as a good thing,as loyalty and honor, other times it is shown as a horrible thing, other times, its just natural. So, what is revenge? Why are humans revengeful? What leads them to revenge? What is the purpose of revenge? Many more questions pop up into my mind and I don't really know how to respond them or even in which way to approach them. Many other topics are also inscribed in our minds in the same way, propaganda makes sure this occurs. So we go around expressing ideas and acting in ways that are not really our own, it is what we infer we should do, and we do it because it is what we think we should do.
To me, how can such a terrible act be such a satisfaction to others? How can revenge be sweet if it only harms others? But then again, I myself have sometimes experienced this satisfaction obtained from revenge, from watching others suffer. Obviously it is not in a big scale, it will just be in a tiny matter or I feel satisfaction because "Karma" is on my side, but the point is still the same. Why do humans have that evil nature in which we can even enjoy somebody else's suffering?