We don't know much about the backstory of Brett, Marvin, and the gang, but we do know they tried to double cross Marsellus Wallace—and nobody double crosses Marsellus Wallace and gets away with it. They pay a steep price for taking Marsellus' briefcase, or not giving him their briefcase…or whatever was supposed to happen.
Nothing says "I've made some poor life decisions recently" than having hamburgers and soda for breakfast. The guys are noshing on Kahuna Burgers at 7:30 AM when their world is invaded by Marsellus' avengers. Jules kills one of them right away just to make a point. Then Brett, whose concentration is a little iffy due to his dead friend lying on the couch, is verbally tortured by Jules before being shot in the shoulder and executed shortly thereafter by Vincent and Jules.
The young man hiding in the bathroom has an inflated idea of what he's capable of, as he empties his gun at the hitmen and misses every shot. He's shocked to see them still standing there. He's only shocked for a few seconds, though.
Then he's dead.
The crazy part about this awful murder scene is that it's comical. The burgers, Jules' extended riff about Marsellus, the bathroom guy's stunned expression after the missed shots—it's funny. The same is true of Marvin's death. He's just riding around in the back of a car one minute and the next he's got his head blown off. His blood is everywhere and there are bits of brain and skull scattered around the back seat. Violent? Yes. Gross? Yes. Funny? Absolutely.
It's this sort of dark and incredibly violent humor that's one of Tarantino's specialties.
Talk talk talk boom.