@DrewDad,
OK, I just had a bit of fridge horror....
If the black goo is a bioweapon designed to create "perfect killing machines" whenever it's introduced into an ecosystem...
...and they introduced it to Earth...
...and humans became the dominant top-level predator....
...then we're the "perfect killing machines" created by the Engineers...
...and that's why the Engineers want to kill off humanity.