Are people jumping to 2 conclusions or did I miss something?

I saw the film last night at 12:01am - good movie, light on answers. Two things that struck me though 1. The orignal engineer that drunk the black stuff. There seems to be a general assumption that this was all part of an initial organised sacrifice to kick start life on earth. Not sure thats ever confirmed though, and he could quite as easily be someone working alone, perhaps even just killing himself (because he missed the last bus home) without actually knowing the consequences. This might explain why the others want to come back later and kill everyone to clean up after him. If it was an organised way of kick starting life then it seems a bit haphazard method to say the least. If he'd have fell the other way he would have just disintegrated on rocks rather than falling into the water? Surely there would be a more controlled way of doing it, if it was part of an scientific process? 2. The other thing, in the movie and in the forums that people just take as read was that the engineers wanted to get back to earth to kill everyone. Where does that come from? There is proof they have been to earth many times before and not killed everyone. Each time they could have been carrying the black goo or other methods of destroying all life but they didn't use it - what makes everyone so sure they are going to this time? (and if they are coming to destroy all life it seems again an odd way of doing it, introducing the black goo, which appears quite random in its effect on different organisms - with life on earth so very varied, who is to say how it would react with birds, cats, insects, fish etc? Again surely there would be a more organised way to wipe out all life for such an advanced civilisation? Infact maybe their new purpose was not to destroy at all? The original SJ that disolved in the water after drinking the black goo actually have a very positive effect, presumable starting life as we know it - the black stuff did not go on to create a planet of violent hace hugging acid blooded monsters. Either he didn't know what would happen, or thought it was a good thing - but now because they are on their way back everyone assumes that its to destroy everything. Can't work out if its story writing at its most deep and inspired or just very very lazy.