Engineer's on search to 'heal' their race?

So everyone is wondering why the engineers would wipe out the entire population of planets ( earth), wait thousands of years for one race to become superior and then do eradicate them again. it occured to my mind that it could be possible that the engineers are a dying race, all of them are looking the same, there are no (or none showed) females and the living engineer was dazzled by seeing Shaw, what was someone interpreted as him seeing female the first time. so its quiet easy: the engineer race is dying, thes just survive by cloning themselves over and over again. to 'heal' them they plant their dna on planets, hoping that superior race they are creating will turn out as 'healthy' engineers, female and male, so they can reproduce themselves naturally. if the outcome of evolution is not similar to them they wipe it out with the xenomorph and start over. they have to different 'teams' to accomplish this, one is flying across the universe, seeding life, the other one follows the path of the first one, checking planet for planet, if their plan succeded, if not they release the xenos. this would explain the two kinds of spaceships and the enormous amount of black goo they are carrying. excuse the mistakes and please let me know what you think about my theory. ;)