Complex sentence: Their day escalates into an eruption of violence and rage that can only be understood by the two girls, after they brutally stab an elderly woman, they later say purely just for "fun", which is where the films title originated.
Split into simpler sentences: Their day escalates into an eruption of violence and rage that can only be understood by the two girls, until they brutally stab an elderly woman. They later say it was purely just for "fun", which is where the film's title originated.