Motorcycle Boy is shot dead by the police near the end of the story.  

Rumble Fish is basically one big flashback. The story begins with Rusty and Steve Hays randomly meeting each other at a beach five or six years after the main events of the book take place. Simply seeing Steve reminds Rusty of the events that eventually led to his brother's death, and the book then flashes back.

Rusty's brother is Motorcycle Boy, and near the story's close, Motorcycle Boy goes into the pet shop where a couple of Siamese fighting fish are kept. Motorcycle Boy calls them "rumble fish" because the two fish would likely try to kill each other if they were placed together. He wonders briefly if the fish would actually do that if they were set free. Motorcycle Boy decides to break them out of the pet store and set them free in the river. Just as he is about to get to the river, Officer Patterson shoots and kills him. Rusty is told that the shot was meant to be a warning shot, but Rusty does not believe that. Officer Patterson had been hassling Motorcycle Boy for years.

The Motorcycle Boy recognizes the symbolism in the rumble fish--the Siamese fighting fish--that inhabit the pet store. He knows that if they are allowed to be together, they will kill each other; yet, he decides they deserve to be set free. So, he and Rusty-James decide to break into the pet store and release the animals. The Motorcycle Boy carries a bowl of the rumble fish, rushing to the river in order to release them; but before he can do so, he is shot by Officer Patterson, the cop who has hassled him for years. The Motorcycle Boy dies in the street, as do the fish, and Rusty-James is arrested. 

They tell me there was a warning shot. How did they expect him to hear a warning shot when everybody knew he was deaf half the time?... I was there when they turned him over, and he was smiling...