The black goo may not have come from the Aliens, but the biological similarities between them and the hammerpedes, trilobites, and Deacons is unmistakable. As for the mural, I'll give you that, since from what we know of Engineer artwork, the mural might have been a warningl. That being said, you cannot deny that this implies a deep cultural significance for the urns placed there beyond simply being utilitarian:
Now maybe the head is just the equivalent of a flag, but it does show a concept of power projection and that the Engineers were unashamed of their creations. Regardless, all the evidence pointed to the fact that you didn't just wake one up randomly and hope to have a pleasant conversation.
