With regard to the Black Liquid the dead Engineers infected with it simply

The Engineers may well have been running as quickly as possible to get back to the Orrery or the place where the giant head is that has all of the viles stored on the ground to get the vacscine to reverse their now infected bodies which subsequently exploded after a short exposure to it. They may well all be running from one of their own whom had injested the liquid sometime earlier then they all had and so it took to him/her a lot quicker and for its effects to kick-in before theirs did. By the time they all realise what may well become of them too its too late, they already know that they are all infected either knowingly or without their knowledge until the penny drops and they run for 'salvation' in the form of a cure or the vacscine back inside the Temple Head chamber. The reson the last straggler falters just prior to the wall seal coming down upon him is possibly because the infection began to take affect at that moment as was the case with Charlie or maybe he was deliberately let the seal come down on him to prevent his infecting the inner chamber anymore much like Vickers flame-throwed Charlie to prevent his exposing the rest of the entire ship to the virus. The Engineer who stumbles and falls was perhaps displaying the first simptoms of the black substance trying to mutate him and he falls over onto his hands and knees just as Charlie did. but what you cant see and what I wonder is if one of the Engineers out of view already inside the Head Temple chamber shot at the hapless, strickend Engineer which makes him topple down to his knees then over onto his stomach and some merciless Engineer activated the switch which brought that seal down upon his bretherens head.