The Engineer at the beginning of the movie

Very good point. I think it could be something like were we were a mistake but maybe there is a bit more to us and that why he was going to complete his mission. But makes sense.

I also think that ingesting the goo was supposed to make him stronger and able to withstand environments in preparation for setting up a base which was what i think the original Engineer on earth was going to do. That's why we see reference of ingesting causing mutation in strength and mass with 2 of the characters. When passed via sexually though is causes physical mutation. I think the engineers also all seemed male from my observations, worms on the other hand are hermaphrodites and so can change sex at will. This would explain their mutation into something different and what can also explain the beginnings of the alien life cycle.

I've seen it as some kind of suicide... as he was dressed like he was following some kind of ritual... so maybe all of them didn't agree on what was going on on that planet...

From the way he appeared to be abandoned, one could speculate that they are referencing another Greek story... Socrates drinking hemlock. The punishment for this Space Jockey is to be ripped apart at genetic level, (the alternative being to starve alone on an unoccupied earth) and thus seeding the planet. There's enough signs there to suggest a death cult amongst them, and that perhaps that opening scene was also a teaser to be answered in the next instalment. (If it was just to plant DNA, surely they wouldn't have to kill one of their own, when they could just as easily grow lots of cells in their equivalent of a petri dish). There should be some sort of meaning behind the deliberate death of this character, whether he knew he would die this way, or whether he was forced into the role of the sacrificial 'seed'.

I think the engineer at the beginning drank the liquid and jumped in the river in order to infect the other Engineers on the base. We can see the DNA being carried away by the flowing water....and this is how it get's transmitted…by drinking it...The humanoid infect that guy in the space ship by dipping his finger in his drink... So the engineer at the beginning was probably trying to stop them all from taking off and going to destroy the Earth. It worked and all of them died... but one. What do you think?

Actualy no...Ignore what i said above. I would agree with the idea that he was creating life on the planete and that the way they do it. After all, life came from the Oceans this is why he had to fall in the river...

Good point. As a summary I tend to believe they chose a solitary planet and 1 single dude to carry out a test, alone (explaining the ritual). The other planet prometheus landed on had more than one ship but was clearly not their home planet.