Revealing mistake: When Trevor is stabbed, we can see the knife isn't in him, it's just pressed against his shirt.
Continuity mistake: When Eugene goes to see Arlene at the casino, there is one guy with a green flower shirt who opens the door and leaves twice.
Chosen answer: The one who stabbed him was not. The others were there because they did not mean for Trevor to get stabbed and killed so now they feel guilty. They were showing Arlene they were sorry.
They were sorry for killing him.
Only one of the bullies killed him. The others were bullying, but didn't think the other would kill him. However, they still had remorse and were showing that remorse to the mother.