A Failed Mission for the SJ

I have been speculating about something and welcome opinion: The engineers decided to destroy life on earth 2000 years ago - reason unknown at this stage - and we know that the derelict on LV-426 has been there for throusands of years so...... Is it realistic to assume that (for whatever reason) the engineers dispatch a lone SJ in a ship with a cargo of their most destructive life form bound for earth, but on the way something goes wrong - a rogue facehugger or something impregnates the SJ, who realising that he won't survive long enough to reach earth lands on LV-426 to protect the cargo but also activates a warning beacon to alert other non human travellers to be prepared. On LV-223 something also goes wrong and the facility is wholly shut down and sealed awaiting perhaps a rescue or antidote etc. The SJ there is placed in cryo sleep warm in the knowledge that the cargo is on its way to earth - only to be awoken by humans and their creation David - who may actually have told him your plan failed right before he took his head off (unless anyone speaks fluent space jockey I guess we won't know until the next film!) If the intention was to destroy human life on earth then the SJ on LV-223 must not let them leave hence the rage. Opinions??????