A weapon?

So I've been thinking about this, and the notion that the ship with the ampules with the black liquid was a weapon designed to kill humans makes ZERO sense. I mean, just think about it! We have an advanced alien race capable of interstellar travel, and in order to wipe us puny humans out they have to send a ship with a few ampules containing a black liquid that will somehow result in the spawn of the xenormorph which will somehow act as a bioweapon and exterminate humans... all this instead of just nuking the crap out of us xD. The xenomorph CANNOT be a bioweapon. I personally think the xenomorph (and the black liquid etc.) play a role in some complex life cycle that we don't have a complete image of yet. In Prometheus, I recall that the engineers DNA matched human DNA, but there's a difference between humans and engineers, so maybe engineers are another stage of this complex life cycle? It must be noted though that due to some very obvious flaws (like the removal of the helmets), Prometheus as a movie isn't reliable regarding scientific accurary, so this could mean nothing. Regardless, I still think that the xenomorph plays a much bigger role than simply being some hazardous bioweapon, and ideally I would want to see a ton of horrifying creatures in Prometheus 2!