BERNARDO: More gracious living. Every one of you hates every one of us, and we hate you right back.
Bernardo has his faults, but a lack of self-awareness isn't one of them. He sees that the Jets and the Sharks hate each other. He just doesn't think that dynamic can change, and frankly, he wouldn't much want to change even if he could.
JETS: The Jets are in gear, Our cylinders are clickin'! The Sharks'll steer clear 'Cause every Puerto Rican's A lousy chicken!
From the very beginning, revenge for slights both real and imagined is in the air. The Jets are going to stick it to the Sharks because they feel the Sharks have been sticking it to them. Nothing else matters in their minds.
JETS: We're gonna rock it tonight, We're gonna jazz it up and have us A ball.
SHARKS: They're gonna get it tonight; The more they turn it on, the Harder they'll fall!
JETS: Well, they began it—
SHARKS: Well, they began it—
BOTH GANGS: And we're the ones to stop 'em Once and for all. Tonight!
This "stop 'em once and for all" thing is what keeps tripping the gangs up. It never ends. It just creates this cycle of revenge that leaves people on both sides dead and anger levels higher than ever. It even consumes Tony, who was smart enough to opt out of it a long time ago.
ANITA: A boy like that wants one thing only, And when he's done he'll leave you lonely. He'll murder your love; he murdered mine. Just wait and see, Just wait, Maria, Just wait and see!
We do wait and see, and while Maria's tempted to commit revenge the same way that Anita is, she just doesn't make that leap. This scene's important because it states very clearly that revenge is still a choice, not something that's forced on you.
ACTION: We've gotta show them who's on top!
JOYBOY: The Jets!
ACTION: Let's do it now!
You can go through all the justifications for revenge: the elaborate reasons, the feeling that the world will end if you don't settle the other guys' hash. But at the end of the day, it all comes down to petty power dynamics: you need to show everyone who's boss. We're supposed to leave that logic on the 3rd grade playground. Unfortunately, we don't.
MARIA: If Chino hurts him, if he touches him, I swear to you I'll...
ANITA: You'll do what Tony did to Bernardo?
MARIA: I love Tony.
ANITA: I know. I loved Bernardo.
Anita's warning is more than just selfish. She's saying that this business is getting bloody and that loving Tony may be a luxury Maria can't afford. I mean, what if some crazy fool blows Tony's head off?
ICE: Boy, boy, crazy boy Get cool, boy! Got a rocket, in your pocket Keep coolly cool, boy! Don't get hot, 'Cause man, you got Some high times ahead. Take it slow and, Daddy-o, You can live it up and die in bed!
Ice must have heard that popular phrase, "revenge is a dish best served cold." Ice fully intends to get even with the Sharks. He just needs to do it without the cops getting involved. And that means all of the Jets have to play it cool. They'll have time for payback once everyone calms down.
ANITA: Bernardo was right. If one of you was lying in the street bleeding, I'd walk by and spit on you.
Anita didn't start out this vindictive. She got pushed into it from all the hate from the Jets. Yes, she makes a choice, but that choice would be a whole lot easier if the Jets weren't trying to assault her.
ANITA: Don't you touch me! I got a message for your American buddy. You tell that murderer that Maria's never going to meet him. You tell him that Chino found out about them and shot her. She's dead.
Taking revenge can be a lot easier that shooting someone dead. It can be just planting the seeds of a bad idea, spreading false information… and then waiting for the hate that's already in the air to do the job for you.
MARIA: All of you! You all killed him! And my brother, and Riff. Not with bullets, or guns, with hate. Well now I can kill, too, because now I have hate!
Yeah, Maria's angry. But unlike the Jets and the Sharks, she doesn't take revenge, Not on Chino and not on anyone else. She simply tells them all what they have done, and completely shreds the "getting even" excuse that most of them have been using to excuse their behavior.