"Blood for Blood, Brother for Brother"

The above title is a very good example of why the Jets and Sharks not only fought with each other, but also retaliated on behalf of any of their "brothers" who were attacked and and even killed physically by the opposing gang. This is a common thing that goes on throughout the world, generally, . as well as here in the United States. The Middle East is still like that today, and is similar to the way that Northern Ireland used to be, during the Troubles, as well.

The constant tit-f0r-tat smiting and killing goes on worldwide, and West Side Story portrays that, not only during the skirmishes between the Jets and the Sharks on the playground, but especially during the Rumble, especially when the Shark gang leader, Bernardo, stabs Jet gang leader, Riff (who's also Tony's best friends, and like a brother to him) to death. Not surprisingly, Tony retaliates by stabbing Bernardo to death during the Rumble, admitting that Riff was like his brother, so he retaliated by stabbing Bernardo to death during the Rumble, as well.

This sort of "blood for blood" and "brother for brother" kind of quarrel also continued when Chino, out of anger and jealousy, ultimately shot and killed Tony, both in retaliation for Tony's having stabbed and killed Bernardo during the Rumble, and out of jealousy and anger, and in retaliation for Tony's having fallen in love with Maria.

Yes, it's a rather sad, unfortunate state of affairs, but the world, in reality is like that, West Side Story is fiction, but it's also close to reality, in some ways, as well.