it's quite easy to put a dent in the "poor good alien" narrative: stand around doing nothing. once enough time passes, biters will kill you irregardless. mind it does take a while for them to come get you, but they will come: https://i.imgur.com/GFxUvu4.png

so the engineer motive is clear: survival, not invasion.

besides, the aliens are an infestation, not part of the ecosystem; as a matter of fact, alien left to their own device will expand and kill the planet biomass. aliens are there to consume, no less than the engineer.