Theories I've heard put forth (and apologies if this isn't the most coherent / elegantly phrased. I'm tired, hungry, and grumpy. :P ):

Book - he used to be an operative similar to the one that was hunting River in the movie. It'd explain how he knew about the weapons and other military-y stuff during Firefly, and how he knew belief was key.

Wash - Someone had to be killed off to make the end scenes work - because he dies, you believe that there's a significant chance they may not make it out of that one alive. Coincidentally, the flight scene immediately prior to his death was my favorite scene in the move.