Continuity mistake: Rachel's father kills himself after returning home from the Vietnam war (presumably in the early 70's) addicted to morphine. The problem with this is that he kills himself in a 1978 or '79 Ford pickup - Ford did not produce any pick-ups with square headlights until those years.
Continuity mistake: In one of the final scenes where Nelson puts himself under, before he injects himself, he doesn't put the suction pads all over his face and chest to track his vitals. Yet when his friends come in, the first thing they check is the EKG machine to check his vitals, which it is displaying. After this, they show Nelson again, and he has the suction pads on his body. How did they get there?
Continuity mistake: When Rachel's father supposedly kills himself in the truck there is a golf ball size hole in the windshield. If he'd used a gun with large caliber the windshield would be destroyed and there would be hair, skull and brain on the windshield, not just blood.
Continuity mistake: When we first see David, he has an argument with Nelson as he packs his stuff into his truck. At the end of the argument, David pulls the rope down, pulling the canvas tight and closing the back of the truck. There is a shot of Nelson, then back to David who pulls on the rope and closes the back of the truck again.
Answer: Billy forgave Nelson. he saw that Nelson was willing to die to be forgiven. That's why Billy smiled.
MasterOfAll