The first scene is this er...Engineer, with a spaceship leaving the surface of this planet, that drinks this strange black liquid, then you see the cells being disrupted, he dies and falls in the water, and from it's dead cells the life starts evolving, with the mitosis spreading everywhere. It means, that dead alien gave us life. This is obviously taken by the Greek myth of Prometheus. It's likely unknown where they come from.
And...who knows? Maybe they thought a biological weapon was better than a chemical one.
Yes, and...?