Engineer vs Deacon

The engineers didn't die from things bursting out of their chests. The holes were in their helmets (and heads), which exploded like the one the humans took to the Prometheus. Being decaptitated stopped that one's head from exploding, so it was a contagion or other agent in their bodies that would explode their heads after inflicting agonizing pain that would bring them to their knees. That was what the one decapitated was doing. He wasn't shot, and Shaw comments that he seemed to be trying to figure out what went wrong. One engineer who was probably seen going through this told the others to RUN!, only it was too late. The lethal agent could have been airborne, though gone by the time the Prometheus came, or maybe their food was poisoned. Clearly, there was a lot of conflict here, which I wish the movie explained more.