Act I · the dance and the rumble
The Jets and Sharks compete for territory. Riff brings Tony, a former Jet, to the gym dance. Tony meets Maria, Bernardo's sister, and the two later meet again on the fire escape. The gangs agree to fight. Maria asks Tony to stop it. Tony tries to intervene, but Bernardo kills Riff and Tony retaliates by killing Bernardo. This sequence matters because Tony's response is a choice made after Riff's death.
Act II · the message
Maria learns what Tony has done and plans to leave with him. Anita agrees to carry a message, but at the drugstore the Jets harass and assault her. In anger she tells them that Chino has killed Maria. Tony hears the false report and calls out to Chino. Maria appears alive. Chino shoots Tony before the two can escape.
Why Maria survives
Maria confronts the groups after Tony dies. The fact that she survives leaves someone on stage able to describe the costs of what has happened. In Shakespeare, the final reconciliation is explained among surviving witnesses after both lovers die. Here the surviving lover brings the accusation directly to the people who helped create the conditions for violence.
A chain of decisions
Map the ending as decisions rather than accidents: the gangs agree to fight, Tony retaliates, the Jets attack the messenger, Anita gives false information and Chino pulls the trigger. Which decision could change the ending while leaving the rest of the story intact? The answer can differ depending on whether you are analysing individual responsibility or the group pressures shaping it.