The Ending
I've read many comments from people who don't find the ending to the film
believable. Most of them have trouble believing that Mario would behave so
recklessly or childishly. IMHO, this is exactly as he was bound to behave by
the very laws established in the film. Remember when they're recruiting
the drivers earlier in the film and they tell them what the job is, one man
backs out saying he's seen men who've done the same job turned old and white
from fear. Later in the film Jo chides Mario for his lack of caution saying
he (Jo) has imagination and brains and thus knows the consequences of recklessness.
Meanwhile, Mario laments that Jo doesn't have guts as well as
brains. In fact, ever since Jo's display of cowardice, Mario has treated him with scorn and contempt.
No trace of their former comeraderie remains. This scene finishes with Jo commenting that Mario's gut will be strewn
from the trees like dead leaves.
Thus, I read the ending of the film as Mario's final vanquishing of fear. He's come through his ordeal, the sole survivor,
and he's filled with euphoria. He feels indestructible, perhaps immortal. And it is precisely this absence of fear which
amounts to a disregard of his own mortality that brings about the fall
of the protagonist of the story.