I'm pretty sure that the Alien shown at the end was the first ever species, and it goes back to the theme of creating a "perfect" creature. David was considered a "perfect" creation, and after rewatching Alien last night, the species created at the end of Prometheus is considered "absolute DNA perfect". So from my stand point it seems like David poisoned that bro, so that he would impregnate Rapace with the tentacle beast, which would thru "divine providence" save her in the end and create the perfect being (the Alien).