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I assumed he was Prometheus by letting the virus out on the planet and killing the Engineers before they could take off and kill earth.
One question though:
The opening scene showed an 'engineer' willingly drinking the bio-weapon and then dying. I guess he was some kind of 'suicide bomber' who was willing to die to infect that planet through its water? Or was he creating life on that planet using his dna interacting with the bio-engineered weapon?