Heres another theory, engineers wanted to wipe out humanity,, they were becoming few in numbers, and with not enough engineers to control what they created they decide to destroy., why? Because we are hostile and there would be no way for them to defend themswlves if we came looking.. on the other hand im sure they tried to tweak the black goo into something they could benefit from. but it went wrong.. the infection turned bad killing off what was left of them 2,000 years ago. To use it on humanity would mean they could study it more after they see what it can do to us. Also destroying us could be to protect their work since they slowly dying out.