Andy, Trevor, and Billy are childhood friends.
As young adults, Billy sells drugs and Andy does deliveries for him.
One night, Trevor delivers instead and gets busted by narcs.
5 years later, Trevor is out and learns his girlfriend Laura is now with Billy.
Trevor gets back with her and decides to rob a local mobster with Andy for some quick cash to buy her stuff. They quickly get caught by the head mobster, who knew Andy's grandfather, so he has them work off what they owe rather than kill them. They do this by smuggling stolen diamonds.
Billy finds out about this and about Trevor re-stealing his girlfriend, so he convinces the mobsters that Trevor is a junkie and is stealing from the mob. He also has a diamond dealer give Trevor fake diamonds to give to the mobsters, who think that Trevor switched them.
Billy figured the mobsters would send a goon to kill Trevor. Instead, they want Billy to do the killing. He shows up at Laura's house and kidnaps Trevor. He takes Trevor to a field and pulls a gun on him. Even though he struggles with his conscience briefly, he ultimately pulls the trigger, killing Trevor.
Later, Andy and Laura get worried about Trevor's disappearance and snoop around. They learn what happened and Andy decides to do something about it. He goes to Billy's house with a gun and pulls it on Billy. Billy tries to bully Andy into putting down the gun (since his bullying Andy has worked so well in the past). Their conversation goes something like this:
Billy: Don't you know how tight I am with the mafia? If you kill me, you'll get yourself killed!
Andy: I'll take my chances BOOM! Andy kills Billy.
In the last scene, Andy has taken Trevor's motorcycle and it's implied that he'll be travelling to California (which was Trevor's intention after he got out of prison).