Both Jets and Sharks bear responsibility for loss of lives on both sides:

A rumble was eminent from the beginning in West Side Story. All the skirmishes between the Jets and Sharks, as well as the showdown at the dance invariably led up to that, in the end, and it was bound to lead to the loss of lives.

Had there been no loss of lives, and people on both sides were seriously injured, but survived, I could and would say that the newly-arrived Puerto Rican Sharks were defending themselves and fighting for their right to a piece of terrain, as well.

Since lives were lost on both sides, however, I have to say that both the Jets and the Sharks bear responsibility for the loss of lives. Riff, Tony and Bernardo, who had a history of gang leadership and the perpetration of violence, all three of them ultimately ended up on the receiving end of it, with the subsequent loss of their lives.

What I noticed at the end of the original 1961 film version of West Side Story is that Maria, despite being devastated by the loss of Tony, walked off in the procession together, after several Jets and Sharks came together to help carry Tony's body off, thus presenting a ray of hope, and possible reconciliation and the development of friendships between the Jets and Sharks. The interesting thing about this film version of West Side Story is that it leaves possibilities wide open to the audiences' imagination.