When Tony leaves the Jets, it falls to Riff to lead them, and frankly speaking, they could do a lot worse. Riff's a juvenile delinquent —he has a whole song about it—but he's having so much fun. Just look at his acrobatics on the playground, dancing and strutting.
"Fun," in fact, seems to be Riff's watchword. He enjoys being a street punk. Not specifically to hurt people (which would be much less fun), but because it tells him who he is.
King of the Hill
Riff is defined by his Jet-ness. The gang represents all he is and all he aspires to. It gives him a sense of belonging, some good buddies to hang out with, and even a sense of personal worth that a street kid from New York could never hope for. He sings all about it in the movie's opening number:
RIFF: When you're Jet
You're a Jet all the way
From your first cigarette
To your last dying day.
When you're a Jet
Let 'em do what they can.
You've got brothers around,
You're a family man!
Being king of the hill, even the cruddy little hill of his cruddy little neighborhood, makes Riff feel good. In that opening dance number, and frankly in the rest of the film too, he's smiling, happy and clowning around, laughing, swinging on the playground, and generally enjoying life. He views himself as something of a benevolent dictator, and he's not even particularly mean. He just needs everyone to understand that the Jets call the shots.
Look at the way he deals with the kids on the basketball court in the beginning: holding onto their ball and letting them know he can do whatever he wants to with it. It's a jerk move, menacing and disruptive, but he gives it back instead of keeping it or punting it onto a nearby rooftop. He was in it for respect, not to ruin everyone else's day. Even when he's out for blood, the dude has a smile on his face.
That's why the Sharks rub him the wrong way. They don't knuckle under. They fight back. In Riff's mind, they disrespect him, and in the process, they ruin his good time. When he's around them, he morphs right into tough-guy mode; no smiles. The Sharks have to be out of the picture before he can think of himself as the king of the hill again.
Like a Brother
Riff is 100% loyal to Tony. They started the gang together, and when the Jets question Tony's character—he took a job, for Pete's sake—Riff isn't hearing any of it.
ACTION: Tony don't belong any more.
RIFF: Cut it, Action boy. I and Tony started the Jets. […] He's always come through for us and he will now.
We learn later that Riff's been living with Tony for more than four years. So he's really like an adoptive brother, and he knows Tony's got his back no matter what.
Naturally We're Punks
There's a reason Riff's moved in with Tony's family. He jokes about it, but you don't leave home unless things are pretty bad. Riff has some insight into what makes him who he is, why he only feels at home in the gang. In "Officer Krupke," he's comically serious:
RIFF: Dear kindly Sergeant Krupke,
You gotta understand.
It's just our bringin' up-ke
That gets us outta hand.
Our mothers all are junkies,
Our fathers all our drunks.
Golly Moses, naturally we're punks!
It's a great scene, hugely funny and playful. But Riff nails it. You could argue he's just making excuses for the Jets' behavior, but he's speaking truth to power. Lt. Schrank knows it. The Jets know it. Their lives are a mess.
Because Riff's character is so energetic and animated, his death comes as a shock. Right up until the moment that Bernardo stabs him at the rumble, we feel like things have to be all right. How can someone so full of life come to such a sudden end? Maybe because he's just from the wrong side of the tracks. No one in his neighborhood ever gets away clean.
At least he died a Jet. Like he told Tony, "Womb to tomb; birth to earth."