Love is a beautiful emotion and it's difficult to put this feeling into exact words to define it. Revenge, an abstract emotion, can not be explained in words. But surprisingly, revenge is also a form of love.
Consider a boy who wholeheartedly was in love with a girl. Things went well until her father got her married to another boy. The boy who was in love with the girl was meek enough to go against her father. After her marriage, the girl was not at all happy with her husband who used to drink and beat her. Upon hearing this, the boy sought a plan to get his love back. He planned and got her husband killed eventually. The girl is now a widow and finding her sad in this condition her father got her married to her lover. This way the two former lovers could get united again.
Now, the above anecdote rightly suggests that had the guy not killed her husband, his love would have failed in the eyes of not only himself but even the rest of the world who think that love is all about caring. He had sacrificed his love, but when she was not happy in her married life because of the ill-treatment, revenge by the guy only helped her to be joyous.